The Ambac Financial Group, Inc., generally known as Ambac (originally the American Municipal Bond Assurance Corporation), is an American holding company. Its subsidiaries provide financial guarantee products such as bond insurance and other financial services to clients in both the public and private sectors around the world. Its common stock and common stock purchase warrants are listed on the Nasdaq stock market under the symbols AMBC and AMBCW respectively. Ambac is regulated by the insurance commission of Wisconsin. Through its financial services subsidiaries, the company provides investment agreements, interest rate swaps, investment advisory and cash management services, primarily to states, municipalities and their authorities. It has its headquarters in Lower Manhattan, New York City.

Ambac and other bond guarantors such as MBIA were hit hard by the 2007 subprime mortgage financial crisis and on January 18, 2008, its Fitch credit rating was lowered from AAA (the highest) to AA when its plans to raise two billion dollars in new capital failed. Moody's and S&P, however, chose to affirm Ambac's AAA with their agencies after it succeeded in raising $1.5 billion in new capital in March 2008. In early 2008, the spectre of the major bond guarantors failing to be able to pay off insurance claims on a trillion dollars of securities backed by sub-prime mortgages and other securitized debt led to attempts to shore them up with infusions of capital. On June 19, 2008 Moody's downgraded Ambac's credit rating three notches to Aa3.

Ambac's ratings continued to decline during 2008–2010. Moody's Investors Service lowered the rating of the senior unsecured debt of Ambac Financial Group Inc. (Ambac Financial) to C from Ca and placed the Caa2 insurance financial strength ratings (IFSR) of Ambac Assurance Corporation (AAC) on review for possible upgrade on 3/26/2010. On March 25, 2010, Standard & Poor's Ratings Services revised its counterparty credit, financial strength and financial enhancement ratings on Ambac Assurance Corp. (Ambac) to 'R' from 'CC'. An 'R' rating is a non-investment grade rating. On June 8, 2010 Ambac announced that it would likely seek a pre-packaged bankruptcy as it was unable to pay dividends from its bond insurance unit to the holding company. Since that time Ambac has dropped appreciably in share price to approximately 0.50 per share as of the end of August 2010.

Ambac missed an interest payment on its debt on November 1, 2010 and filed for chapter 11 bankruptcy protection on November 8, 2010.

Ambac exited Chapter 11 on May 1, 2013, issuing 45 million new common shares and approximately 5 million new warrants to holders of allowed claims. Nader Tavakoli, who later became Chief Executive Officer, was on Ambac’s Board of Directors during the time of the registration statement.

After serving as interim President and CEO, and then Executive Chairman in 2015, the Board formally appointed Tavakoli to the position of President and CEO in January 2016. Tavakoli helped lead a turnaround during his tenure that included "substantial progress" in areas such as "risk and loss analytics, operational efficiency, asset management, shareholder communication, and loss management, including our major loss recovery cases."

Tavakoli was succeeded by current CEO Claude LeBlanc in 2017, who said upon assuming the role that "I look forward to working with Ambac's Board, talented management team, dedicated employees and primary regulator to continue executing the company's strategies to optimize assets and reduce liabilities, while maintaining a clear focus on risk and regulatory matters."