Ancient Aliens is an American television series produced by Prometheus Entertainment that explores the pseudoscientific hypothesis of ancient astronauts in a non-critical, documentary format. Episodes also explore related pseudoscientific and pseudohistoric topics, such as: Atlantis and other lost ancient civilizations, extraterrestrial contact and ufology, and popular conspiracy theories. The series, which has aired on History since 2010, has been a target for criticism of History's channel drift, as well as criticism for promoting unorthodox or unproven hypotheses as fact. According to Smithsonian, episodes of the series overwhelm the viewer with "fictions and distortions" by using a Gish gallop.

Originally broadcast as two-hour documentary special in 2009, Ancient Aliens: The Series aired for three seasons as a flagship series on History from 2010 to 2012. The series moved to H2 from 2012 to 2014, with frequent re-airings of episodes on History and other A&E services. In 2015, the series returned to History after H2 was relaunched as Vice on TV. A nineteenth season began in 2023. All episodes are narrated by Robert Clotworthy.

The series is inspired by the works of Erich von Däniken, Zecharia Sitchin, Graham Hancock, Robert Bauval, Brinsley Trench, Charles Hapgood, and Edgar Cayce. Producer Giorgio Tsoukalos, writer David Childress and journalist Nick Pope are featured guests.

The series has been criticized by historians, cosmologists, archaeologists and other scientists for presenting and promoting pseudoscience, pseudohistory and pseudoarchaeology as fact. Episodes are frequently characterized as "far-fetched", "hugely speculative", and "expound[ing] wildly on theories suggesting that astronauts wandered the Earth freely in ancient times." Many of the claims made by guests are not commonly accepted as fact by the scientific community. Brian Dunning debunked the series as "a slap in the face to the ingenuity of the human race".