The American subprime mortgage crisis was a multinational financial crisis that occurred between 2007 and 2010, contributing to the 2008 financial crisis. It led to a severe economic recession, with millions becoming unemployed and many businesses going bankrupt. The U.S. government intervened with a series of measures to stabilize the financial system, including the Troubled Asset Relief Program (TARP) and the American Recovery and Reinvestment Act (ARRA).

The collapse of the United States housing bubble and high interest rates led to unprecedented numbers of borrowers missing mortgage repayments and becoming delinquent. This ultimately led to mass foreclosures and the devaluation of housing-related securities. The housing bubble preceding the crisis was financed with mortgage-backed securities (MBSes) and collateralized debt obligations (CDOs), which initially offered higher interest rates (i.e. better returns) than government securities, along with attractive risk ratings from rating agencies. Despite being highly rated, most of these financial instruments were made up of high-risk subprime mortgages.

While elements of the crisis first became more visible during 2007, several major financial institutions collapsed in late 2008, with significant disruption in the flow of credit to businesses and consumers and the onset of a severe global recession. Most notably, Lehman Brothers, a major mortgage lender, declared bankruptcy in September 2008. There were many causes of the crisis, with commentators assigning different levels of blame to financial institutions, regulators, credit agencies, government housing policies, and consumers, among others. Two proximate causes were the rise in subprime lending and the increase in housing speculation. Investors, even those with "prime", or low-risk, credit ratings, were much more likely to default than non-investors when prices fell. These changes were part of a broader trend of lowered lending standards and higher-risk mortgage products, which contributed to U.S. households becoming increasingly indebted.

The crisis had severe, long-lasting consequences for the U.S. and European economies. The U.S. entered a deep recession, with nearly 9 million jobs lost during 2008 and 2009, roughly 6% of the workforce. The number of jobs did not return to the December 2007 pre-crisis peak until May 2014. U.S. household net worth declined by nearly $13 trillion (20%) from its Q2 2007 pre-crisis peak, recovering by Q4 2012. U.S. housing prices fell nearly 30% on average and the U.S. stock market fell approximately 50% by early 2009, with stocks regaining their December 2007 level during September 2012. One estimate of lost output and income from the crisis comes to "at least 40% of 2007 gross domestic product". Europe also continued to struggle with its own economic crisis, with elevated unemployment and severe banking impairments estimated at €940 billion between 2008 and 2012. As of January 2018, U.S. bailout funds had been fully recovered by the government, when interest on loans is taken into consideration. A total of $626B was invested, loaned, or granted due to various bailout measures, while $390B had been returned to the Treasury. The Treasury had earned another $323B in interest on bailout loans, resulting in an $109B profit as of January 2021.

The immediate cause of the crisis was the bursting of the United States housing bubble which peaked in approximately 2006. An increase in loan incentives such as easy initial terms and a long-term trend of rising housing prices had encouraged borrowers to assume risky mortgages in the anticipation that they would be able to quickly refinance at easier terms. However, once interest rates began to rise and housing prices started to drop moderately in 2006–2007 in many parts of the U.S., borrowers were unable to refinance. Defaults and foreclosure activity increased dramatically as easy initial terms expired, home prices fell, and adjustable-rate mortgage (ARM) interest rates reset higher.

As housing prices fell, global investor demand for mortgage-related securities evaporated. This became apparent by July 2007, when investment bank Bear Stearns announced that two of its hedge funds had imploded. These funds had invested in securities that derived their value from mortgages. When the value of these securities dropped, lenders demanded that these hedge funds provide additional collateral. This created a cascade of selling in these securities, which lowered their value further. Economist Mark Zandi wrote that this 2007 event was "arguably the proximate catalyst" for the financial market disruption that followed.

Several other factors set the stage for the rise and fall of housing prices, and related securities widely held by financial firms. In the years leading up to the crisis, the U.S. received large amounts of foreign money from fast-growing economies in Asia and oil-producing/exporting countries. This inflow of funds combined with low U.S. interest rates from 2002 to 2004 contributed to easy credit conditions, which fueled both housing and credit bubbles. Loans of various types (e.g., mortgage, credit card, and auto) were easy to obtain and consumers assumed an unprecedented debt load.

As part of the housing and credit booms, the number of financial agreements called mortgage-backed securities (MBS), which derive their value from mortgage payments and housing prices, greatly increased. Such financial innovation enabled institutions and investors around the world to invest in the U.S. housing market. As housing prices declined, major global financial institutions that had borrowed and invested heavily in MBS reported significant losses. Defaults and losses on other loan types also increased significantly as the crisis expanded from the housing market to other parts of the economy. Total losses were estimated in the trillions of U.S. dollars globally.