Ancient astronauts (or ancient aliens) refers to a pseudoscientific set of beliefs, also called paleocontact, that hold that intelligent extraterrestrial beings (alien astronauts) visited Earth and made contact with humans in antiquity and prehistoric times. Proponents of the theory suggest that this contact influenced the development of modern cultures, technologies, religions, and human biology. A common position is that deities from most (if not all) religions are extraterrestrial in origin, and that advanced technologies brought to Earth by ancient astronauts were interpreted as evidence of divine status by early humans.

The idea that ancient astronauts existed and visited Earth is not taken seriously by academics and archaeologists, who identify such claims as pseudoarchaeological or unscientific. It has received no credible attention in peer-reviewed studies. When proponents of the idea present evidence in favor of their beliefs, it is often distorted or fabricated. Some authors and scholars also argue that ancient astronaut theories have racist undertones or implications, diminishing the accomplishments and capabilities of indigenous cultures.

Well-known proponents of these beliefs in the latter half of the 20th century who have written numerous books or appear regularly in mass media include Robert Charroux, Jacques Bergier, Jean Sendy, Erich von Däniken, Alexander Kazantsev, Zecharia Sitchin, Robert K. G. Temple, Giorgio A. Tsoukalos, David Hatcher Childress, Peter Kolosimo, and Mauro Biglino.

Various terms are used to reference claims about ancient astronauts, such as ancient aliens, ancient ufonauts, ancient space pilots, paleocontact, astronaut- or alien gods, or paleo- or biblical-SETI (search for extraterrestrial intelligence).

Believers in such ancient astronaut stories often maintain that all or some humans are either descendants or creations of extraterrestrial intelligence who landed on Earth at some point in the ancient past. An associated idea is that human knowledge, religion, and culture came from extraterrestrial visitors in ancient times, in that ancient astronauts acted as a "mother culture". Additionally, proponents often claim that travelers from outer space built many of the structures on Earth (such as Egyptian pyramids and the Moai stone heads of Easter Island) or aided humans in building them. Proponents contend that the evidence for ancient astronauts comes from documentary gaps in historical and archaeological records while citing archaeological artifacts that they believe, contrary to the mainstream explanations, are anachronistic and supposedly beyond the technical capabilities of the people who made them. These are sometimes referred to as "out-of-place artifacts"; and include artwork and legends which believers reinterpret to fit stories of extraterrestrial contact or technologies.

As a pseudoarcheology, the idea receives notice in fringe pulp media, such as the History Channel series Ancient Aliens. Such shows use a strategy known as 'fire-hosing' to co-mingle fact with fiction in order to spread theories of an alternate past with tropes that follow white supremacist, nativist, imperialist, settler-colonial, and Christian Identity beliefs relevant to the past. The celebrity proponents of ancient aliens profess to be a part of an oppressed minority of academics that 'big archaeology' is conspiring to disenfranchise while their identity of being a maverick or a rogue aligns with the individuals' lack of credentials.

Like archaeological endeavors of the criticized past, these proponents focus primarily on monumental archaeological structures claiming they could have only been constructed with extraterrestrial intervention. The implication is that the non-white Indigenous people in the regions in which these monuments appear could not have built them on their own. However, Dakota/Lakota Sioux writer Ruth H. Burns, in Atmos magazine, counters that ancient alien theory and the idea of extraterrestrials in general supports the viewpoints of indigenous, non-European peoples. She believes the denial of extraterrestrial encounters and indigenous peoples' stories tracing their origins to extraterrestrials is part of "Indigenous erasure," as it minimizes or completely discounts the viewpoints of indigenous peoples. Many indigenous peoples trace their ancestry to "star-people" or the like—extraterrestrials who as the progenitors of indigenous peoples cannot by definition be white or "Aryan."

A common feature in the stories portray the aliens as light-skinned or Aryan in complexion, as prominent alien astronaut proponent Erich von Däniken claims in his foundational work Chariots of the Gods? Some ancient astronaut proponents are thus associated with white supremacism, although their theories are sometimes applied to European cultures as well.