TIMELY INFORMATION INSIDE

Strategic Total Return Fund (CSQ)

SEMIANNUAL REPORT APRIL 30, 2018

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TABLE OF CONTENTS

Letter to Shareholders1

The Calamos Closed-End Funds:
An Overview5

Investment Team Discussion6

Schedule of Investments10

Statement of Assets and Liabilities26

Statement of Operations27

Statements of Changes In Net Assets28

Statement of Cash Flows29

Notes to Financial Statements30

Financial Highlights38

Report of Independent Registered
Public Accounting Firm39

About Closed-End Funds40

Managed Distribution Policy41

Automatic Dividend Reinvestment Plan41

Experience and Foresight

About Calamos Investments

For over 40 years, we have helped investors like you manage and build wealth to meet their long-term individual objectives by working to capitalize on the opportunities of the evolving global marketplace. We launched our first mutual fund in 1985 and our first closed-end fund in 2002. Today, we manage six closed-end funds. Three are total-return oriented offerings, which seek current income, with increased emphasis on capital gains potential. Three are enhanced fixed income offerings, which pursue high current income from income and capital gains. Calamos Strategic Total Return Fund (CSQ) falls into the former category. Please see page 5 for a more detailed overview of our closed-end offerings.

We are dedicated to helping our clients build and protect wealth. We understand when you entrust us with your assets, you also entrust us with your achievements, goals and aspirations. We believe we best honor this trust by making investment decisions guided by integrity, by discipline, and by our conscientious research.

We believe an active, risk-conscious approach is essential for wealth creation. In the 1970s, we pioneered strategies that seek to participate in equity market upside and mitigate some of the potential risks of equity market volatility. Our investment process seeks to manage risk at multiple levels and draws upon our experience investing through multiple market cycles.

We have a global perspective. We believe globalization offers tremendous opportunities for countries and companies all over the world. In our view, this creates significant opportunities for investors. In our U.S., global and international portfolios, we are seeking to capitalize on the potential growth of the global economy.

We believe there are opportunities in all markets. Our history traces back to the 1970s, a period of significant volatility and economic concerns. We have invested through multiple market cycles, each with its own challenges. Out of this experience comes our belief that the flipside of volatility is opportunity.

John P. calamos, sr.

Founder, Chairman
and Global Chief
Investment Officer

CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT   1

Dear Fellow Shareholder:

Welcome to your semiannual report for the six-month period ended April 30, 2018. In this report, you will find commentary from the Calamos portfolio management team, as well as a listing of portfolio holdings, financial statements and highlights, and detailed information about the performance and positioning of this Calamos Fund. I encourage you to review this information carefully.

Calamos Strategic Total Return Fund (CSQ) is an income-oriented total return fund. This means we are focused not only on delivering a competitive stream of distributions, but also on total return. We utilize dynamic asset allocation to pursue high current income with a less rate-sensitive approach, while also maintaining a focus on capital gains.

Distribution

During the period, the Fund provided a compelling monthly distribution of $0.0825 per share. We believe the Fund’s current annualized distribution rate, which was 8.27%* on a market price basis as of April 30, 2018, was very competitive, given the low interest rates in many segments of the bond market. In our view, the Fund’s distributions illustrate the benefits of a multi-asset class approach and flexible allocation strategy.

We understand that many closed-end fund investors seek steady, predictable distributions instead of distributions that fluctuate. Therefore, this Fund has a managed distribution policy. As part of this policy, we aim to keep distributions consistent from month to month, and at a level that we believe can be sustained over the long term. In setting the Fund’s distribution rate, the investment management team and the Fund’s Board of Trustees consider the interest rate, market and economic environment. We also factor in our assessment of individual securities and asset classes.

*Current Annualized Distribution Rate is the Fund’s most recent distribution, expressed as an annualized percentage of the Fund’s current market price per share. The Fund’s 4/30/18 distribution was $0.0825 per share. Based on our current estimates, we anticipate that approximately $0.0825 is paid from ordinary income or capital gains and that approximately $0.0000 represents a return of capital. Estimates are calculated on a tax basis rather than on a generally accepted accounting principles (GAAP) basis, but should not be used for tax reporting purposes. Distributions are subject to re-characterization for tax purposes after the end of the fiscal year. This information is not legal or tax advice. Consult a professional regarding your specific legal or tax matters. Under the Fund’s managed distribution policy, distributions paid to common shareholders may include net investment income, net realized short-term capital gains and return of capital. When the net investment income and net realized short-term capital gains are not sufficient, a portion of the distribution will be a return of capital. In addition, a limited number of distributions per calendar year may include net realized long-term capital gains. The distribution rate may vary.

2   CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT

In our October 2017 annual report, I discussed the favorable trends that were supporting continued global growth and investment potential across asset classes. However, I noted our investment teams would not be surprised if the market environment changed, with a pause in the equity markets or even a correction. I also discussed that short-term volatility can create opportunities for long-term managers, such as ourselves. During the semiannual reporting period, these views were borne out.

Market Review

The semiannual period was characterized by two distinct environments, and in particular, the return of volatility to the markets in January. Through the final months of 2017, U.S. and global stock markets advanced briskly as investors focused on positive global economic growth trends, low inflation, deregulation and anticipation of tax reform. However, the climate changed in 2018. Although economic fundamentals remained compelling around the world, market participants became distracted by the potential for more Federal Reserve hikes and rising inflation. After many years of subdued volatility and record-low interest rates, the stock market became increasingly turbulent as the period progressed.¹ Fixed income asset classes also encountered headwinds.^2,3 Convertible securities, in contrast, demonstrated resilience in the choppy markets, illustrating the potential benefits of their unique structural characteristics.⁴

Outlook

We believe positive economic conditions and corporate fundamentals can support a continuation of the bull market in stocks, as well as opportunities across asset classes. Global economic data is positive on the whole. GDP growth has continued at a good pace in the U.S., and tax reform and deregulation can provide further catalysts from here. The U.S. consumer has benefitted from job growth, wage gains and rising housing values. Corporate fundamentals—including earnings, sales and revenue growth—are very strong. Inflation has increased but is coming off historically low levels and remains contained. As economic growth continues, we expect the Fed to maintain its course of raising short-term interest rates gradually. This slow pace, combined with conditions in the broader global bond markets, may help keep longer-term U.S. rates from rising unchecked.

Although economic conditions are favorable, managing volatility will be key to succeeding in this environment. Evolving trade policies, uncertainties around North Korea and Russia, and U.S. mid-term elections will likely contribute to short-term market disruptions over coming months. An extended trade war could be an economic headwind, but we do not see this as the probable outcome. We are hopeful that moves by the U.S. over recent months can set the stage for negotiations that lead to freer and fairer trade over the long term.

CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT   3

Thoughts for Investors

The stock market’s gyrations, rising interest rates and signs of inflation have unsettled many investors. However, if we look back over longer periods, the environment we are seeing today is actually more normal than those of the past few years, when volatility, interest rates and inflation were exceptionally low.

In this more typical environment, we believe there are many opportunities for active and risk-aware approaches. Our investment professionals remain vigilant to avoiding unintended risks in the Fund. They are also working together to turn short-term volatility into opportunities for the shareholders in the Fund. Our ability to respond to changing market conditions will be especially important given the many crosscurrents we see.

In Closing

More than 40 years of experience supports my belief that investors are best served by maintaining a long-term approach. Fast moving markets can increase the temptation to make short-term moves. But as I’ve noted in the past, there’s no way to predict with certainty the daily twists and turns. Far too often, trying to time the ups and downs leaves investors capturing the downside and missing the upside. Instead, I encourage you to work with your financial advisor to create a well-diversified asset allocation that meets your financial goals. By blending an array of funds—including stocks, convertibles, alternative and fixed income strategies—investors may be better positioned to achieve their long-term goals.

I am confident that the Fund is well equipped to navigate the course ahead. Since our earliest days as an asset manager, we have sought to provide innovative strategies for managing risk and enhancing returns. While others may be discouraged by these more choppy markets, we welcome the return of more normal levels of volatility and the opportunities it will bring for our active approach.

I hope that you find this report informative, and I invite you to visit our website www.calamos.com for additional resources. All of us at Calamos Investments thank you for your continued trust. It is truly an honor to help you achieve your financial goals.

Sincerely,

John P. Calamos, Sr.

Founder, Chairman and Global Chief Investment Officer

4   CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT

Before investing, carefully consider a fund’s investment objectives, risks, charges and expenses. Please see the prospectus containing this and other information or call 800.582.6959. Please read the prospectus carefully. Performance data represents past performance, which is no guarantee of future results. Current performance may be lower or higher than the performance quoted. Opinions are as of the publication date, subject to change and may not come to pass. Information is for informational purposes only and shouldn’t be considered investment advice.

¹The MSCI All Country World Index is a measure of global stock market performance, which returned 3.83% for the six-month period ending April 30, 2018. The MSCI World Index is a market-capitalization weighted index composed of companies representative of the market structure of developed market countries in North America, Europe and the Asia/Pacific region. For the six-month period ended, the index returned 3.68%. The S&P 500 Index is an unmanaged index generally considered representative of the U.S. stock market. For the six-month period, the index returned 3.82%. The MSCI Emerging Markets Index is a free float-adjusted market capitalization index considered broadly representative of emerging market equity performance. The index represents companies within the constituent emerging markets that are available to investors worldwide, and it returned 4.93% for the six-month period ended April 30, 2018.

²The Bloomberg Barclays U.S. Aggregate Bond Index is considered generally representative of the investment-grade bond market. For the six-month period ending April 30, 2018, the index returned -1.87%.

³The Credit Suisse U.S. High Yield Index is an unmanaged index of high yield debt securities, which returned -0.16% for the six-month period ending April 30, 2018.

⁴The ICE BofAML All U.S. Convertibles ex-Mandatory Index represents the U.S. convertible securities market excluding mandatory convertibles. The index returned 2.34% for the six-month period ending April 30, 2018. The ICE BofAML Global 300 Convertible Index is a global convertible index composed of companies representative of the market structure of countries in North America, Europe and the Asia/Pacific region. The index returned 3.95% for the six-month period ended April 30, 2018.

Sources: Lipper, Inc.

Unmanaged index returns assume reinvestment of any and all distributions and, unlike fund returns, do not reflect fees, expenses or sales charges. Investors cannot invest directly in an index. Investments in overseas markets pose special risks, including currency fluctuation and political risks. These risks are generally intensified for investments in emerging markets. Countries, regions, and sectors mentioned are presented to illustrate countries, regions, and sectors in which a fund may invest. Fund holdings are subject to change daily. The Funds are actively managed.

The information contained herein is based on internal research derived from various sources and does not purport to be statements of all material facts relating to the securities mentioned. The information contained herein, while not guaranteed as to accuracy or completeness, has been obtained from sources we believe to be reliable. There are certain risks involved with investing in convertible securities in addition to market risk, such as call risk, dividend risk, liquidity risk and default risk, that should be carefully considered prior to investing. This information is being provided for informational purposes only and should not be considered investment advice or an offer to buy or sell any security in the portfolio. Investments in alternative strategies may not be suitable for all investors.

This report is intended for informational purposes only and should not be considered investment advice.

CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT   5

In our closed-end funds, we draw upon decades of investment experience, including a long history of opportunistically blending asset classes in an attempt to capture upside potential while seeking to manage downside risk. We launched our first closed-end fund in 2002.

Closed-end funds are long-term investments. Most focus on providing monthly distributions, but there are important differences among individual closed-end funds. Calamos closed-end funds can be grouped into multiple categories that seek to produce income while offering exposure to various asset classes and sectors.

Our Managed Distribution Policy

Closed-end fund investors often seek a steady stream of income. Recognizing this important need, Calamos closed-end funds adhere to a managed distribution policy in which we aim to provide consistent monthly distributions through the disbursement of the following:

•Net investment income

•Net realized short-term capital gains

•Net realized long-term capital gains

•And, if necessary, return of capital

We set distributions at levels that we believe are sustainable for the long term. Our team focuses on delivering an attractive monthly distribution, while maintaining a long-term emphasis on risk management. The level of the funds’ distributions can be greatly influenced by market conditions, including the interest rate environment, the individual performance of securities held by the funds, our view of retaining leverage, fund tax considerations, and regulatory requirements.

You should not draw any conclusions about the Fund’s investment performance from the amount of this distribution or from the terms of the Fund’s plan. The Fund’s Board of Trustees may amend or terminate the managed distribution policy at any time without prior notice to the Fund’s shareholders.

For more information about any Calamos closed-end funds, we encourage you to contact your financial advisor or Calamos Investments at 800.582.6959 (Monday through Friday from 8:00 a.m. to 6:00 p.m., Central Time). You can also visit us at www.calamos.com.

6   CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT

TOTAL RETURN* AS OF 4/30/18

SECTOR WEIGHTINGS

Sector Weightings are based on managed assets and may vary over time. Sector Weightings exclude any government/sovereign bonds or options on broad market indexes the Fund may hold.

Strategic Total Return Fund (CSQ)

INVESTMENT TEAM DISCUSSION

Please discuss the Fund’s strategy and role within an asset allocation framework.

Calamos Strategic Total Return Fund (CSQ) is a total-return-oriented offering that seeks to provide a steady stream of income paid out on a monthly basis. We invest in a diversified portfolio of equities, convertible securities and high-yield securities. The allocation to each asset class is dynamic, and reflects our view of the economic landscape as well as the potential of individual securities. By combining these asset classes, we believe that we are well positioned over the long term to generate capital gains as well as income. This broader range of security types also provides us with increased opportunities to manage the risk/reward characteristics of the portfolio over full market cycles. Through this approach, we seek to offer investors an attractive monthly distribution and equity participation.

We believe our diversified exposure to the equity markets enables us to manage the Fund’s risk during high periods of market volatility and allows us to optimize our performance on an ongoing basis.

While we invest primarily in securities of U.S. issuers, we favor those companies that are actively participating in globalization with geographically diversified revenue streams and global business strategies. We emphasize companies that we believe offer reliable debt servicing, respectable balance sheets, solid free cash flow and good prospects for sustainable growth. We continue to remain generally optimistic with respect to the U.S. equity market in the short-term and look forward to continued growth through economic expansion and reflationary market dynamics, accompanied by the benefits of global economic synchronization.

How did the Fund perform over the reporting period?

The Fund increased 3.16% on a net asset value (NAV) basis and 1.10% on a market price basis for the six-month period ended April 30, 2018. The S&P 500 Index finished at 3.82% and the Credit Suisse U.S. High Yield Index dipped -0.16% for the same period.

At the end of the reporting period, the Fund’s shares traded at a -5.30% discount to net asset value.

CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT   7

SINCE INCEPTION MARKET PRICE AND NAV HISTORY THROUGH 4/30/18

Performance data quoted represents past performance, which is no guarantee of future results. Current performance may be lower or higher than the performance quoted. The principal value of an investment will fluctuate so that your shares, when sold, may be worth more or less than their original cost. Returns at NAV reflect the deduction of the Fund’s management fee, debt leverage costs and all other applicable fees and expenses. You can obtain performance data current to the most recent month end by visiting www.calamos.com.

How do NAV and market price return differ?

Closed-end funds trade on exchanges, where the price of shares may be driven by factors other than the value of the underlying securities. The price of a share in the market is called market value. Market price may be influenced by factors unrelated to the performance of the fund’s holdings, such as general market sentiment or future expectation. A fund’s NAV return measures the actual return of the individual securities in the portfolio, less fund expenses. It also measures how a manager was able to capitalize on market opportunities. Because we believe closed-end funds are best utilized long term within asset allocations, we believe that NAV return is the better measure of a fund’s performance. However, when managing the fund, we strongly consider actions and policies that we believe will optimize its overall price performance and returns based on market value.

Please discuss the Fund’s distributions during the six-month period.

We employ a managed distribution policy* within this Fund with the goal of providing shareholders a consistent distribution stream. In each month of the period, the Fund distributed $0.0825 per share, resulting in a current annualized distribution rate of 8.27% of market price as of April 30, 2018.

We believe that both the Fund’s distribution rate and level remained attractive and competitive, as low interest rates limited yield opportunities in much of the marketplace. For example, as of April 30, 3018, the dividend yield of S&P 500 Index stocks averaged approximately 2.11%.† Yields also were low within the U.S. government bond market, with the 10-year U.S. Treasury and 30-year U.S. Treasury yielding 2.95% and 3.11%, respectively.‡

*Under the Fund’s managed distribution policy, distributions paid to common shareholders may include net investment income, net realized short-term capital gains and return of capital. When the net investment income and net realized short-term capital gains are not sufficient, a portion of the distribution will be a return of capital. In addition, a limited number of distributions per calendar year may include net realized long-term capital gains. The distribution rate may vary.

†Source: Calamos Investments

‡Source: U.S. Department of the Treasury

8   CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT

What factors influenced performance over the reporting period?

The Fund’s wide investment parameters allow us to take advantage of investment ortunities through numerous types of investment vehicles. By taking advantage of such flexibility, the Fund maintained its exposure to the equity markets, which improved during the period. In addition, both convertibles and bonds provided income and appreciation to the portfolio for the six months.

In terms of sectors and corresponding industries, our selection in utilities, specifically in electric utilities, contributed to performance relative to the S&P 500 Index. In addition, our slight underweight to consumer staples, notably tobacco, was helpful during the period.

Selection in financials (regional banks) detracted from the Fund’s performance relative to the S&P 500 Index for the period. Our selection in consumer discretionary (internet and direct marketing retail) also lagged relative to the index.

How is the Fund positioned?

We continue to favor businesses with compelling growth opportunities and global revenue drivers. We believe an enhanced focus on risk management and valuation sensitivity is beneficial. We also believe that the U.S. economy still has room to grow as investors contemplate reflationary economic forces, tax cuts, consumer optimism, and lower regulations, which all have the potential to drive equity price valuations.

We are also cognizant of the fact that rising interest rates can have a detrimental effect on longer-term fixed income securities. Consequently, managing the duration of the fixed-income assets of our portfolio is a priority in mitigating the potential impact.

In terms of positioning, we maintained a relatively high allocation to U.S. stocks and convertibles, whose combined exposure is approximately 75% of the portfolio. We are excited about the opportunities in the convertible market as steady issuance offers additional choices in the space. Accordingly, we have been able to take advantage of attractive opportunities and establish meaningful exposure in those investments.

The portfolio currently holds large absolute allocations to the information technology, consumer discretionary and financials sectors. We expect our positioning to benefit in the future as more companies seek solutions to enhance and improve productivity and business performance. We believe these sectors remain poised to benefit from many of our long-term secular themes, including robust consumer demand for products and services that provide access to information. We believe consumer discretionary stocks will also benefit from improving economic conditions derived from increasing employment, improved housing valuations, and growing consumer confidence. We have also maintained our position in financials, as we expect the sector will benefit during periods of rising interest rates, increased lending, and a more hospitable regulatory environment.

CALAMOS Strategic Total Return Fund SEMIANNUAL REPORT   9

The average credit quality of the portfolio is higher than that of the Credit Suisse U.S. High Yield Index. This is typical for the Fund, as our credit process tends to guide us away from the most-speculative corporate securities. That said, we recognize that opportunities are available for lower-credit securities to enhance performance.

We believe that this is an environment that is conducive to the prudent use of leverage as a means of enhancing total return and supporting the fund’s distribution rate. In spite of a cost increase due to rising interest rates, our use of leverage over the period enjoyed a favorable reinvestment dynamic.

What is your outlook for the Fund?

In our view, equities continue to offer compelling risk/reward characteristics, and we are seeking companies with attractively valued fundamentals. We have also seen continued issuance in the convertible markets, which present additional opportunities to generate income while participating in equity market appreciation. Equity and equity-sensitive securities continue to offer attractive risk/reward characteristics from a long-term perspective. We continue to pursue our strategic objective to participate in a significant portion of equity market upside while aiming to reduce volatility versus global equities throughout a complete market cycle.

What are your closing thoughts for Fund shareholders?

Given our outlook for a near-term period of economic growth, we are favoring quality growth companies. Favorable factors within the U.S. include solid job creation, low interest rates, increased consumer and corporate confidence and limited inflationary pressures. We are emphasizing investments in companies with solid cash-flow generation and stronger balance sheets that will benefit from global economic synchronization. From a thematic and sector perspective, we see opportunities in the technology sector, consumer discretionary companies tied to global consumption, and companies positioned to benefit from improving fundamentals. We are also optimistic about financials, as we believe that many companies in the sector are favorably valued and positioned to grow revenues in a rising interest rate and lower regulatory environment. We are selective regarding companies in the health care sector, favoring those that are more product-growth oriented. We are cautious about companies in the consumer staples sector, which may be fully valued as investors seek those stocks for income rather than growth.

We believe that fiscal policy is likely to remain an important factor regarding the impact of rate hikes on corporate refinancing of debt. Geopolitical concerns will also be a factor adding to already increased volatility. In our view, equities continue to offer compelling risk/reward characteristics, and we seek companies with attractively valued fundamentals. With increased volatility, however, active management is imperative to both managing risk and optimizing opportunities.