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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 10-K

For the fiscal year ended December 31, 2005

OR

For the transition period from              to            

Commission file number 001-31978

Assurant, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code:

(212) 859-7000

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act:

None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  x    No  ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨     No  x

Note—Checking the box above will not relieve any registrant required to file reports pursuant to Section 13 or 15(d) of the Exchange Act from their obligations under those Sections.

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of the registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    Yes  ¨    No  x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, or a non-accelerated filer. See definition of “accelerated filer and large accelerated filer” in Rule 12b-2 of the Exchange Act. (Check One):

x    Large accelerated filer        ¨    Accelerated filer        ¨    Non-accelerated filer

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act).    Yes  ¨    No  x

The aggregate market value of the Common Stock held by non-affiliates of the registrant was $4,078 million at June 30, 2005 based on the closing sale price of $36.10 per share for the common stock on such date as traded on the New York Stock Exchange.

The number of shares of the registrant’s Common Stock outstanding at March 1, 2006 was 129,959,069.

Documents Incorporated by Reference

Certain information contained in the definitive proxy statement for the annual meeting of stockholders to be held on May 18, 2006 (2006 Proxy Statement) is incorporated by reference into Part III hereof.

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ASSURANT, INC.

ANNUAL REPORT ON FORM 10-K

For the Fiscal Year Ended December 31, 2005

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FORWARD-LOOKING STATEMENTS

Some of the statements under “Business,” “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” and elsewhere in this report may contain forward-looking statements which reflect our current views with respect to, among other things, future events and financial performance. You can identify these forward-looking statements by the use of forward-looking words such as “outlook,” “believes,” “expects,” “potential,” “continues,” “may,” “will,” “should,” “seeks,” “approximately,” “predicts,” “intends,” “plans,” “estimates,” “anticipates” or the negative version of those words or other comparable words. Any forward-looking statements contained in this report are based upon our historical performance and on current plans, estimates and expectations. The inclusion of this forward looking information should not be regarded as a representation by us or any other person that the future plans, estimates or expectations contemplated by us will be achieved. Such forward-looking statements are subject to various risks and uncertainties. Accordingly, there are or will be important factors that could cause our actual results to differ materially from those indicated in this report. We believe that these factors include but are not limited to those described under the subsection entitled “Risk Factors” in “Management’s Discussion and Analysis of Financial Condition and Results of Operations.” These factors should not be construed as exhaustive and should be read in conjunction with the other cautionary statements that are included in this report. We undertake no obligation to publicly update or review any forward-looking statement, whether as a result of new information, future developments or otherwise.

If one or more of these or other risks or uncertainties materialize, or if our underlying assumptions prove to be incorrect, actual results may vary materially from what we projected. Any forward-looking statements you read in this report reflect our current views with respect to future events and are subject to these and other risks, uncertainties and assumptions relating to our operations, results of operations, financial condition, growth strategy and liquidity.

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PART I

Item 1. Business

Assurant, Inc. (Assurant) is a Delaware corporation, formed in connection with the Initial Public Offering (“IPO”) of its common stock, which began trading on the New York Stock Exchange (“NYSE”) on February 5, 2004. Prior to that initial trading date, Fortis, Inc., a Nevada corporation, had formed Assurant and merged into it on February 4, 2004. The merger was executed in order to redomesticate Fortis, Inc. from Nevada to Delaware and to change its name. As a result of the merger, Assurant is the successor to the business operations and obligations of Fortis, Inc.

Prior to the IPO, 100% of the outstanding common stock of Fortis, Inc. was owned indirectly by Fortis N.V., a public company with limited liability incorporated as naamloze vennootschap under Dutch law, and Fortis SA/ NV, a public company with limited liability incorporated as société anonyme/naamloze vennootschap under Belgian law. Following the IPO, Fortis N.V. and Fortis SA/ NV, through a wholly owned subsidiary Fortis Insurance N.V., owned approximately 35% (50,199,130 shares) of the outstanding common stock of Assurant.

On January 21, 2005, Fortis N.V. and Fortis SA/ NV, through a wholly owned subsidiary Fortis Insurance N.V., owned approximately 36% (50,199,130 shares) of the outstanding common stock of Assurant based on the number of shares outstanding that day and sold 27,200,000 of those shares in a secondary offering to the public. Assurant did not receive any of the proceeds from the sale of shares of common stock. Fortis N.V. received all net proceeds from the sale and concurrently sold exchangeable bonds, due January 26, 2008, that are mandatorily exchangeable for their remaining 22,999,130 shares of Assurant. The exchangeable bonds and the shares of Assurant’s common stock into which they are exchangeable have not been and will not be registered under the Securities Act of 1933 (“Securities Act”) and may not be offered or sold in the United States absent registration or an applicable exemption from registration requirements.

In this report, references to the “Company,” “Assurant,” “we,” “us” or “our” refer to (1) Fortis, Inc. and its subsidiaries, and (2) Assurant, Inc. and its subsidiaries after the consummation of the merger described above. References to “Fortis” refer collectively to Fortis N.V. and Fortis SA/ NV. References to our “separation” from Fortis refer to the fact that Fortis reduced its ownership of our common stock in connection with the secondary offering.

Dollar amounts are presented in U.S. dollars and all amounts are in thousands, except number of shares, per share amounts, registered holders, number of employees and beneficial owners.

Overview

We pursue a differentiated strategy of building leading positions in specialized market segments for insurance products and related services in North America and selected international markets. We provide creditor-placed homeowners insurance, manufactured housing homeowners insurance, debt protection administration, credit insurance, warranties and extended service contracts, individual health and small employer group health insurance, group dental insurance, group disability insurance, group life insurance and pre-funded funeral insurance.

The markets we target are generally complex, have a relatively limited number of competitors and, we believe, offer attractive profit opportunities. In these markets, we leverage the experience of our management team and apply our expertise in risk management, underwriting and business-to-business management, as well as our technological capabilities in complex administration and systems. Through these activities, we seek to generate above-average returns by building on specialized market knowledge, well-established distribution relationships and economies of scale.

As a result of our strategy, we are a leader in many of our chosen markets and products. In the Assurant Solutions business, we have leadership positions or are aligned with clients who are leaders in creditor-placed homeowners insurance based on servicing volume, manufactured housing homeowners insurance based on number of homes built and debt protection administration based on credit card balances outstanding. In the

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Assurant Health business, we were among one of the first companies to offer Medical Savings Accounts (“MSAs”) and Health Savings Accounts (“HSAs”). In the Assurant Employee Benefits business, we are a leading writer of group dental, disability and life plans sponsored by employers based on the number of subscribers and based on the number of master contracts in force. A master contract refers to a single contract issued to an employer that provides coverage on a group basis; group members receive certificates, which summarize benefits provided and serve as evidence of membership. In the Assurant PreNeed business, we are the largest writer of pre-funded funeral insurance in Canada measured by face amounts of new policies sold and the sole provider of pre-funded funeral insurance to the largest funeral home firm in North America. We believe that our leadership positions give us a sustainable competitive advantage in our chosen markets.

We currently have four operating business segments to ensure focus on critical activities close to our target markets and customers, while simultaneously providing centralized support in key functions. Each operating business segment has its own experienced management team with the autonomy to make decisions on key operating matters. These managers are eligible to receive incentive-based compensation based in part on operating business segment performance and in part on company-wide performance, thereby encouraging strong business performance and cooperation across all our businesses. At the operating business segment level, we stress disciplined underwriting, careful analysis and constant improvement and product redesign. At the corporate level, we provide support services, including finance, legal, litigation compliance, regulatory relations, investment, asset/liability matching and capital management, mergers and acquisitions, treasury, leadership development, information technology support and other administrative functions, enabling the operating business segments to focus on their target markets and distribution relationships while enjoying the economies of scale realized by operating these businesses together. Also, our overall strategy and financial objectives are set and continuously monitored at the corporate level to ensure that our capital resources are being properly allocated.

We organize and manage our specialized businesses through four operating business segments:

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We also have a Corporate and Other segment, which includes activities of the holding company, financing expenses, net realized gains (losses) on investments, interest income earned from short-term investments, interest income from excess surplus of insurance subsidiaries not allocated to other segments and additional costs associated with excess of loss reinsurance programs reinsured and ceded to certain subsidiaries in the London market between 1995 and 1997. The Corporate and Other segment also includes the amortization of deferred gains associated with the portions of the sales of Fortis Financial Group (“FFG”) and Long Term Care (“LTC”) sold through reinsurance agreements. See “Item 7—Management’s Discussion and Analysis of Financial Condition and Results of Operations—Corporate and Other.”

Competition

We face competition in each of our businesses; however, we believe that no single competitor competes against us in all of our business lines and the business lines in which we operate are generally characterized by a limited number of competitors. Competition in our operating business segments is based on a number of factors, including: quality of service, product features, price, scope of distribution, financial strength ratings and name recognition.

The relative importance of these factors depends on the particular product and market. We compete for customers and distributors with insurance companies and other financial services companies in our various businesses.

Assurant Solutions has several competitors in its product lines, but we believe no other company participates in all of the same lines or offers comparable comprehensive capabilities. Competitors include insurance companies and financial institutions. In Assurant Health, we believe the market is characterized by

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many competitors, and our main competitors include health insurance companies, Health Management Organizations (“HMOs”) and the Blue Cross/Blue Shield plans in the states in which we write business. In Assurant Employee Benefits, commercial competitors include other benefit and life insurance companies as well as dental managed care entities and not-for-profit Delta Dental plans. In Assurant PreNeed, our main competitors are several small regional insurers. While we are among the largest competitors in terms of market share in many of our business lines, in some cases there are one or more major market players in a particular line of business.

Competitive Strengths

We believe our competitive strengths include:

Leadership Positions in Specialized Markets. We are a market leader in many of our chosen markets. We seek to participate in markets in which there are a limited number of competitors and that allow us to achieve a market leading position by capitalizing on our market expertise and capabilities in complex administration and systems, as well as on our established distribution relationships. We believe that our leadership positions provide us with the opportunity to generate high returns in these niche markets.

Strong Relationships with Key Clients and Distributors. We have created strong relationships with our distributors and clients in each of the niche markets we serve. We believe that the strength of our distribution relationships enables us to market our products and services to our customers in an effective and efficient manner that would be difficult for our competitors to replicate.

History of Product Innovation and Ability to Adapt to Changing Market Conditions. We are able to adapt quickly to changing market conditions by tailoring our product and service offerings to the specific needs of our clients. This flexibility was developed, in part, as a result of our entrepreneurial focus and the encouragement of management autonomy at each business segment. By understanding the dynamics of our core markets, we design innovative products and services and seek to sustain profitable growth and market leading positions.

Disciplined Approach to Underwriting and Risk Management. Our businesses share best practices of disciplined underwriting and risk management. We focus on generating profitability through careful analysis of risks and by drawing on our experience in core specialized markets. We prioritize loss containment and we purchase reinsurance as a risk management tool to diversify risk and protect against unexpected events, such as catastrophes. We believe that our disciplined underwriting and risk management philosophy have enabled us to realize above average financial returns while executing our strategic objectives.

Prudent Capital Management. We seek to generate above-average returns on a risk-adjusted basis from our operating activities. We invest capital in our operating business segments when we identify attractive profit opportunities in our target markets. To the extent that we believe we can achieve, maintain or improve leadership positions in these markets by deploying our capital and leveraging our expertise and other competitive advantages, we have done so with the expectation of generating high returns. When expected returns have justified continued investment, we have reinvested cash from operations into enhancing and growing our operating business segments through the development of new products and services, additional distribution

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relationships and other operational improvements. In addition, when we have identified external opportunities that are consistent with these objectives, we have acquired businesses, portfolios, distribution relationships, personnel or other resources. We believe we have benefited from having the discipline and flexibility to deploy capital opportunistically and prudently to maximize returns to our stockholders.

Diverse Business Mix and Excellent Financial Strength. We have four operating business segments across distinct areas of the insurance market. These businesses are generally not affected in the same way by economic and operating trends, which we believe allows us to maintain a greater level of financial stability than many of our competitors across business and economic cycles. Our domestic rated operating insurance subsidiaries have financial strength ratings of A (“Excellent”) or A- (“Excellent”) from A.M. Best, six of our domestic operating insurance subsidiaries have financial strength ratings of A2 (“Good”) from Moody’s and seven of our domestic operating insurance subsidiaries have financial strength ratings of A (“Strong”) or A- (“Strong”) from Standard and Poors (“S&P”). We employ a conservative investment policy and our portfolio primarily consists of high grade fixed income securities. We believe our solid capital base and overall financial strength allow us to distinguish ourselves from our competitors and continue to enable us to attract clients that are seeking long-term financial stability. In addition to financial strength ratings, these ratings agencies also rate Assurant, Inc.’s senior notes and commercial paper. See “Item 7—Management’s Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources.”

Experienced Management Team with Proven Track Record and Entrepreneurial Culture. We have a talented and experienced management team both at the corporate level and at each of our business segments. Our management team is led by our Chief Executive Officer, J. Kerry Clayton, who has been with our Company or its predecessors for 35 years. Mr. Clayton plans to retire on March 17, 2006. He will continue to serve on the Company’s Board of Directors until the end of 2006. Robert B. Pollock, who has been with our company for 25 years and is currently Assurant’s President and Chief Operating Officer, has been named President and Chief Executive Officer and has been appointed to the Board of Directors. Our executive officers have an average tenure of approximately 17 years with our Company and close to 20 years in the insurance and related risk management business. Our management team has successfully led our business and executed on our specialized niche strategy through numerous business cycles and political and regulatory challenges. Our management team also shares a set of corporate values and promotes a common corporate culture that we believe enables us to leverage business ideas, risk management expertise and focus on regulatory compliance across our businesses. At the same time, we reward and encourage entrepreneurship at each business segment, accomplished in part by our long history of utilizing performance-based compensation plans.

Growth Strategy

In March of 2005, we announced a strategic growth initiative, emphasizing certain lines of business within each business segment. We established performance goals and short-term incentive compensation for senior management based on the initiatives. The growth initiatives by segment are as follows:

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Operating Business Segments

Our business is comprised of four operating business segments: Assurant Solutions; Assurant Health; Assurant Employee Benefits; and Assurant PreNeed. We also have a Corporate and Other segment. Our business segments and the related net earned premiums and other considerations and fees and other income and segment income before income tax generated by those segments are as follows for the periods indicated:

Net Earned Premiums and Other Considerations and

Fees and Other Income by Segment

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Segment Income (Loss) Before Income Tax by Business Segment