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Annual Report

March 31, 2020

MFS® Municipal Series Trust

For the states of:

Alabama, Arkansas, California, Georgia, Maryland, and Massachusetts

Beginning on January 1, 2021, as permitted by regulations adopted by the U.S. Securities and Exchange Commission, paper copies of the fund’s annual and semiannual shareholder reports will no longer be sent by mail, unless you specifically request paper copies of the reports. Instead, the complete reports will be made available on the fund’s Web site (funds.mfs.com), and you will be notified by mail each time a report is posted and provided with a Web site link to access the report.

If you are already signed up to receive shareholder reports by email, you will not be affected by this change and you need not take any action. You may sign up to receive shareholder reports and other communications from the fund by email by contacting your financial intermediary (such as a broker-dealer or bank) or, if you hold your shares directly with the fund, by calling 1-800-225-2606 or by logging on to MFS Access at mfs.com.

Beginning on January 1, 2019, you may elect to receive all future reports in paper free of charge. Contact your financial intermediary to request that you continue to receive paper copies of your shareholder reports. If you invest directly with the fund, you can call 1-800-225-2606 or send an email request to orderliterature@mfs.com to let the fund know that you wish to continue receiving paper copies of your shareholder reports. Your election to receive reports in paper will apply to all funds held in your account if you invest through your financial intermediary or all funds held with the MFS fund complex if you invest directly.

MSTA-ANN

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MFS® Municipal Series Trust

For the states of: Alabama, Arkansas, California, Georgia, Maryland, and Massachusetts

Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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Portfolio Composition – continued

Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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Portfolio Composition – continued

Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statements of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Other includes equivalent exposure from currency derivatives and/or any offsets to derivative positions.

Percentages are based on net assets as of March 31, 2020.

The portfolio is actively managed and current holdings may be different.

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MANAGEMENT REVIEW

Summary of Results

For the twelve months ended March 31, 2020, Class A shares, at net asset value, of the municipal bond funds of Alabama, Arkansas, California, Georgia, Maryland and Massachusetts underperformed the Bloomberg Barclays Municipal Bond Index. The performance for the individual funds and the benchmark are set forth in the Performance Summary.

Market Environment

Market volatility soared at the end of the period as COVID-19, the illness caused by the novel coronavirus first detected in China, spread around the world in early 2020. Government-imposed social distancing measures taken to slow the spread of the virus forced the shutdown of large portions of the global economy. In an attempt to limit the economic fallout from the crisis, central bankers and policymakers took a variety of actions, unprecedented in both size and speed, but despite their best efforts, a global recession has undoubtedly begun. Given the unique circumstances surrounding the genesis of the downturn, it is difficult to forecast how long the crisis will linger.

High levels of investor uncertainty saw equity prices plunge, market volatility rise and large segments of global fixed income markets freeze as investors sought liquidity wherever they could find it. Central banks around the world quickly put programs in place to improve liquidity, and they proved largely successful, helping to recover market function and ease volatility at the end of the period. Compounding market turmoil was an oil price war between Saudi Arabia and Russia over the size of production cuts amid a burgeoning supply glut as the global economy rapidly slowed. That action undercut oil exporters, many emerging markets and a large segment of the high-yield credit market.

As is often the case in a serious crisis, market vulnerabilities were revealed. For example, companies that added significant leverage to their balance sheets in recent years by borrowing to fund dividend payments and stock buybacks may find it difficult to follow that path in the future, and investors may not reward that behavior if they do. Very long supply chains designed to squeeze out every bit of savings may be shifted closer to home markets if companies seek to emphasize resiliency over cost after a series of supply disruptions. Consumer, business and government behavior could all be impacted by the after-effects of the global pandemic in ways it will be difficult to forecast.

Over the past year, municipal bonds delivered modest positive performance, fueled by lower US Treasury rates and supportive fundamentals and technicals for most of the period. Longer-duration and higher-quality segments of the market outperformed. Each sector produced modest positive performance, with sectors perceived as higher quality outperforming. The favorable backdrop for municipals changed abruptly in March, impacted by the economic slowdown associated with limiting the spread of the coronavirus. The revenues of municipalities were, and continue to be, affected by both declining tax receipts and the increased costs brought on by the response to the outbreak. The fundamentals of many sectors will be impacted by the slowdown. Federal assistance will help in the effort to meet the challenges; however, the selection of sectors and issuers will likely increase in importance over the coming months.

Factors Affecting Performance

In all funds, an out-of-benchmark exposure to non-rated (r) securities weighed on performance relative to the Bloomberg Barclays Municipal Bond Index. The combination of security selection and a greater exposure to Puerto Rican bonds further held back relative performance across all funds.

In contrast, bond selection within the health care sector across all funds, with the exception of the municipal bond fund of Massachusetts, benefited relative performance. Security selection within “AA” and “BBB” rated securities in the municipal bond funds of Alabama, Arkansas, Georgia and Maryland, also aided relative results. Yield curve positioning (y), particularly a greater exposure to shifts in the long end of the yield curve, was another area of relative strength for all funds.

Respectfully,

Portfolio Manager(s)

Michael Dawson

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The views expressed in this report are those of the portfolio manager(s) only through the end of the period of the report as stated on the cover and do not necessarily reflect the views of MFS or any other person in the MFS organization. These views are subject to change at any time based on market or other conditions, and MFS disclaims any responsibility to update such views. These views may not be relied upon as investment advice or an indication of trading intent on behalf of any MFS portfolio. References to specific securities are not recommendations of such securities, and may not be representative of any MFS portfolio’s current or future investments.

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PERFORMANCE SUMMARY THROUGH 3/31/20

The following charts illustrate a representative class of each fund’s historical performance in comparison to its benchmark(s). Performance results include the deduction of the maximum applicable sales charge and reflect the percentage change in net asset value, including reinvestment of dividends and capital gains distributions. Each fund's results have been compared to the Bloomberg Barclays Municipal Bond Index, a market capitalization-weighted index that measures the performance of the tax-exempt bond market. However, while this index is considered the benchmark for the performance of municipal bond funds, it is comprised of municipal bonds issued within the 50 states and the District of Columbia as well as U.S. territories and possessions such as Guam, Puerto Rico, and the Virgin Islands, while each of the funds invests a large percentage of its assets in municipal issuers of the state that is in the fund’s name. The performance of other share classes will be greater than or less than that of the class depicted below. Benchmarks are unmanaged and may not be invested in directly. Benchmark returns do not reflect sales charges, commissions or expenses. (See Notes to Performance Summary.)

Performance data shown represents past performance and is no guarantee of future results. Investment return and principal value fluctuate so your shares, when sold, may be worth more or less than the original cost; current performance may be lower or higher than quoted. The performance shown does not reflect the deduction of taxes, if any, that a shareholder would pay on fund distributions or the redemption of fund shares.

Class I and R6 shares do not have a sales charge.

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Performance Summary – continued

MFS Arkansas Municipal Bond Fund

Growth of a Hypothetical $10,000 Investment

Total Returns through 3/31/20

Average annual without sales charge

Class I and R6 shares do not have a sales charge.

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Performance Summary – continued

MFS California Municipal Bond Fund

Growth of a Hypothetical $10,000 Investment

Total Returns through 3/31/20

Average annual without sales charge

Class I and R6 shares do not have a sales charge.

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Performance Summary – continued

MFS Georgia Municipal Bond Fund

Growth of a Hypothetical $10,000 Investment

Total Returns through 3/31/20

Average annual without sales charge

Class I and R6 shares do not have a sales charge.

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Performance Summary – continued

MFS Maryland Municipal Bond Fund

Growth of a Hypothetical $10,000 Investment

Total Returns through 3/31/20

Average annual without sales charge

Class I and R6 shares do not have a sales charge.

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Performance Summary – continued

MFS Massachusetts Municipal Bond Fund

Growth of a Hypothetical $10,000 Investment

Total Returns through 3/31/20

Average annual without sales charge

Class I and R6 shares do not have a sales charge.

Benchmark Definition(s)

Bloomberg Barclays Municipal Bond Index – a market capitalization-weighted index that measures the performance of the tax-exempt bond market. BLOOMBERG® is a trademark and service mark of Bloomberg Finance L.P. and its affiliates (collectively “Bloomberg”). BARCLAYS® is a trademark and service mark of Barclays Bank Plc (collectively with its affiliates, “Barclays”), used under license. Bloomberg or Bloomberg’s licensors, including Barclays, own all proprietary rights in the Bloomberg Barclays Indices. Neither Bloomberg nor Barclays approves or endorses this material, or guarantees the accuracy or completeness of any information herein, or makes any warranty, express or implied, as to the results to be obtained therefrom, and, to the maximum extent allowed by law, neither shall have any liability or responsibility for injury or damages arising in connection therewith.

It is not possible to invest directly in an index.

Notes to Performance Summary

Average annual total return represents the average annual change in value for each share class for the periods presented. Life returns are presented where the share class has less than 10 years of performance history and represent the average annual total return

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from the class inception date to the stated period end date. As the fund’s share classes may have different inception dates, the life returns may represent different time periods and may not be comparable. As a result, no comparative benchmark performance information is provided for life periods.

Performance results reflect any applicable expense subsidies and waivers in effect during the periods shown. Without such subsidies and waivers each fund’s performance results would be less favorable. Please see the prospectus and financial statements for complete details.

Performance results do not include adjustments made for financial reporting purposes in accordance with U.S. generally accepted accounting principles and may differ from amounts reported in the financial highlights.

A portion of income may be subject to state, federal, and/or alternative minimum tax. Capital gains, if any, are subject to a capital gains tax.

From time to time the funds may receive proceeds from litigation settlements, without which performance would be lower.

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EXPENSE TABLES

Fund expenses borne by the shareholders during the period, October 1, 2019 through March 31, 2020

As a shareholder of the funds, you incur two types of costs: (1) transaction costs, including sales charges (loads) on certain purchase or redemption payments, and (2) ongoing costs, including management fees; distribution and service (12b-1) fees; and other fund expenses. These examples are intended to help you understand your ongoing costs (in dollars) of investing in the funds and to compare these costs with the ongoing costs of investing in other mutual funds.

These examples are based on an investment of $1,000 invested at the beginning of the period and held for the entire period October 1, 2019 through March 31, 2020.

Actual Expenses

The first line for each share class in the following tables provides information about actual account values and actual expenses. You may use the information in this line, together with the amount you invested, to estimate the expenses that you paid over the period. Simply divide your account value by $1,000 (for example, an $8,600 account value divided by $1,000 = 8.6), then multiply the result by the number in the first line under the heading entitled “Expenses Paid During Period” to estimate the expenses you paid on your account during this period.

Hypothetical Example for Comparison Purposes

The second line for each share class in the following tables provides information about hypothetical account values and hypothetical expenses based on each fund’s actual expense ratio and an assumed rate of return of 5% per year before expenses, which is not each fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in each fund and other funds. To do so, compare this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the tables are meant to highlight your ongoing costs only and do not reflect any transactional costs, such as sales charges (loads). Therefore, the second line for each share class in the tables is useful in comparing ongoing costs only, and will not help you determine the relative total costs of owning different funds. In addition, if these transactional costs were included, your costs would have been higher.

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Notes to Expense Table

For the MFS Georgia Municipal Bond Fund, each class with a Rule 12b-1 service fee is subject to a rebate of a portion of such fee. Such rebates are included in the expense ratios above and are outside of the expense limitation arrangement. For Class B shares, this rebate reduced the expense ratio above by 0.02%. See Note 3 in the Notes to Financial Statements for additional information.

For the MFS Maryland Municipal Bond Fund, each class with a Rule 12b-1 service fee is subject to a rebate of a portion of such fee. Such rebates are included in the expense ratios above and are outside of the expense limitation arrangement. For Class A shares, this rebate reduced the expense ratio above by 0.01%. See Note 3 in the Notes to Financial Statements for additional information.

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Notes to Expense Table

For the MFS Massachusetts Municipal Bond Fund, each class with a Rule 12b-1 service fee is subject to a rebate of a portion of such fee. Such rebates are included in the expense ratios above and are outside of the expense limitation arrangement. For Class A shares, this rebate reduced the expense ratio above by 0.02%. See Note 3 in the Notes to Financial Statements for additional information.

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PORTFOLIO OF INVESTMENTS

3/31/20

MFS ALABAMA MUNICIPAL BOND FUND

The Portfolio of Investments is a complete list of all securities owned by your fund. It is categorized by broad-based asset classes.

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