The sewer alligator is a colloquial term for alligators that live in sewers outside alligators' native range. Some cities in which sewer alligators have supposedly been found are New York City and Paris. Accounts of fully grown sewer alligators are unproven, but small alligators are sometimes rescued from sewers. Stories date back to the late 1920s and early 1930s; in most instances they are part of contemporary legend.

The New York Times reports the city rescues several alligators per year, some directly from homes where they are kept as illegal pets (which can be legally ordered online in other states and are legal to mail when small), and some from outside (where they can attract considerable attention) though mostly above-ground. Though escapees and former pets may survive for a short time in New York sewers, longer-term survival is not possible due to the low temperatures and the bacteria in human feces. Sewer maintenance crews insist there is no underground population of alligators in sewers.

The legend of alligators inhabiting the sewer system of New York City is a widely circulated urban myth. It suggests that alligators navigate the city's sewers, preying on rats and other refuse, and posing a threat to sewer workers, who are said to carry firearms for protection. According to the lore, these alligators are often described as large and vicious, with some attributing a lack of pigmentation to their purported status as "albinos." The urban myth has permeated popular culture, featuring in various forms of media including books, television shows, and movies. It has also inspired hoaxes and artistic projects, and is commemorated in the city with a quasi-holiday known as Alligator in the Sewer Day, celebrated on February 9.

Following reports of sewer alligators in the 1930s, the story built up over the decades and became more of a contemporary legend. It is questionable how accurate the original stories are, and some have even suggested they are fictions created by Teddy May, who was the Commissioner of Sewers at the time. Interviews with him were the basis of the first published accounts of sewer alligators. In their honor, February 9 is Alligators in the Sewers Day in Manhattan.

A similar story from 1851 involves feral pigs in the sewers of Hampstead, London.

As late as the middle of the 20th century, souvenir shops in Florida sold live baby alligators (in small fish tanks) as novelty souvenirs. Tourists from New York City would buy a baby alligator and try to raise it as a pet. When the alligator grew too large for comfort, the family would proceed to flush the reptile down the toilet.

The most common story is that the alligators survive and reside within the sewer and reproduce, feeding on rats and garbage, growing to huge sizes and striking fear into sewer workers.^[1] In Robert Daley's book The World Beneath the City (1959) he comments that one night a sewer worker in New York City was shocked to find a large albino alligator swimming toward him. Weeks of hunting followed.

The Journal of American Folklore has this to say on the subject: