The undocumented immigrant population of the United States is the number of resident unauthorized immigrants living on United States territory. The number of such immigrants, and their specific countries of origin, has always been difficult to determine, and an official count of such individuals by the U.S. federal government has never been undertaken. Estimates of this population, derived from national surveys, administrative data, and other sources of information, vary widely. By all measures, the population of undocumented immigrants in the U.S. declined substantially from 2007 until at least 2018. The Pew Research Center reported that the number of border apprehensions declined considerably after 2000, reaching a low in 2017, but then rebounded to reach a new peak level in 2021. In a 2025 report, Pew estimated that the population of undocumented immigrants in the U.S. had risen to 14 million in 2023.

In 1980 the undocumented immigrant population was estimated at 2.06 million. During the 1990s, according to the nonpartisan Pew Research Center, the number rose rapidly; "from an estimated 3.5 million in 1990 to a peak of 12.2 million in 2007," then dropped sharply during the Great Recession before stabilizing in 2009. Pew estimated the total population to be 11.1 million in 2014, or approximately 3 percent of the U.S. population. This "is in the same ballpark" as figures from the United States Department of Homeland Security (DHS), which estimated that 11.4 million undocumented immigrants lived in the United States in January 2012. The estimate and trends are also consistent with figures reported by the Center for Migration Studies, which reported that the U.S. undocumented immigrant population fell to 10.9 million by January 2016, the lowest number since 2003. A 2018 paper by three Yale School of Management professors estimated that the undocumented immigrant population was in the range of 16 million to 29 million; however, the methodology presented in the Yale study has been criticized as leading to vastly overstated results. In a 2016 report, the Pew Research Center had estimated that some 11.1 million undocumented immigrants were living in the U.S. in 2014. In 2025, Pew announced that "the number of unauthorized immigrants in the United States reached an all-time high of 14 million in 2023 after two consecutive years of growth".

The "residual method" is widely used to estimate the undocumented immigrant population of the U.S. With this method, the known number of legally documented immigrants to the United States is subtracted from the reported U.S. Census number of self-proclaimed foreign-born people (based on immigration records and adjusted by projections of deaths and out-migration) to obtain the total undocumented immigrant (residual) population. This methodology is used by the U.S. Department of Homeland Security, the Pew Hispanic Center, the Center for Immigration Studies, and the United States Census Bureau. Since undocumented immigrants have many reasons for not answering the U.S. Census correctly and since penalties for answering the U.S. Census incorrectly are rarely enforced, it is accepted that it under-counts the number of undocumented immigrants. The users of this methodology assume that 10% of undocumented immigrants are not counted by census takers. The 10% assumption is based on a 2001 University of California survey asked of 829 people born in Mexico and living in Los Angeles whether they responded to census interviewers in the 2000 census with 40% of queried households refusing to answer the survey. Critics claim that the estimate is unreliable for a number of reasons: figures for outmigration are not tracked by the federal government; the proportion of undocumented immigrants who respond to the Census is unknown; the estimate that 10% of undocumented immigrants do not respond to the census is arbitrary and unsupported by a sufficient sample size and geographic spread; and that the self-reporting of where one was born relies on the honesty of the person being questioned.

Estimates of this population are generally based on modeling using data from the American Community Survey (ACS) or the Current Population Survey conducted each year by the United States Census Bureau.

A 2018 paper by three Yale School of Management professors yielded similar trajectories of the undocumented immigrant population, with peak growth in the 1990s and early 2000s followed by a plateau from approximately 2008 onward. However, their model yielded an estimate of the numbers of undocumented immigrants of 22.1 million — roughly twice as great as the estimates based on the ACS. Moreover, according to the three authors, their estimate has a 95 percent probability range of 16 million to 29 million. That result, however, was criticized for vastly overstating the true number and for failing to account for the circular flow rate.

The Great Recession had a large impact on the United States. The construction sector and other areas where undocumented immigrants traditionally seek employment shrank. The recession also led to a surplus of American labor, driving down the benefit of hiring undocumented immigrants. According to the Pew Research Center, in 2007 the number of unauthorized Mexican immigrants peaked at 6.9 million and has dropped by more than 1 million to an estimated 5.6 million in 2014.

After the Great Recession, more immigrants actually returned to Mexico rather than migrated to the United States. From 2009 to 2014, 1 million Mexicans and their families left the U.S. for Mexico. U.S. census data for the same period show an estimated 870,000 Mexican nationals left Mexico to return to the U.S. It is hard to track of this because there is no official number of immigrants going to the United States or returning to Mexico every year.

Since about 2014, most undocumented immigrants living in the U.S. have been long-term residents. In 2014, about two-thirds (66%) had been in the U.S. for ten years or more, while just 14% had been in the U.S. for less than five years.