Ancient astronaut theories or paleocontact are various proposals that intelligent extraterrestrial beings have visited Earth and that this contact is linked to the origins or development of human cultures, technologies and/or religions.

Some of these theories suggest that deities from most - if not all - religions are actually extraterrestrial beings, and their technologies were taken as evidence of their divine status.

Ancient astronaut theories have limited support within the scientific community, and have garnered little if any attention in peer-reviewed studies from scientific journals. These theories have been popularized, particularly in the latter half of the 20th century, by writers Erich von Däniken, Zecharia Sitchin and Robert K.G. Temple .

Ancient astronaut theories have been widely used in science fiction.

Ancient astronaut adherents often claim that humans are either descendants or creations of beings who landed on Earth millennia ago. An associated theory is that much of human knowledge, religion and culture came from extraterrestrial visitors in ancient times, in that ancient astronauts acted as a "mother culture". These ideas are generally discounted by the scientific community.

Ancient astronaut theories also may include the idea that civilization may have evolved on earth twice, and that the visitation of ancient astronauts may reflect the return of descendants of ancient humans whose population was separated from earthbound humans.

Proponents of ancient astronaut theories point to what they perceive as gaps in historical and archaeological records, and to what they see as absent or incomplete explanations of historical or archaeological data. Ancient astronaut proponents cite evidence that they argue supports their assertions, notably, archaeological artifacts that they argue are anachronistic or beyond the presumed technical capabilities of the historical cultures with which they are associated (sometimes referred to as "Out-of-place artifacts"); and artwork and legends which are interpreted as depicting extraterrestrial contact or technologies. Examples of such "Out-of-place artifacts" include the Baghdad Battery and the Antikythera mechanism.

Scientists maintain that any gaps in contemporary knowledge of the past do not demonstrate that such speculative ancient astronaut ideas are a necessary, or even plausible, conclusion to draw from the available data. Additionally, a number of ancient astronaut claims contradict consensus scientific interpretations of evidence. The scientific community remains generally skeptical, and the dominant view is that there is no evidence to support ancient astronaut and paleocontact theories.