Ancient astronauts are conjectured extraterrestrial visitors to Earth in its distant past, whose existence was proposed by Erich von Däniken in his book Chariots of the Gods and its sequels. The idea is viewed as pseudoscience by most scientists and historians.

The evidence Däniken has put forward to support his theories can be categorised as follows:

• Artifacts have been found which appear to represent higher technological knowledge than is presumed to have existed at times when they were manufactured. Däniken maintains that these artifacts have been manufactured either by extraterrestrial visitors or by humans who learned the necessary knowledge from them. Such artifacts include Stonehenge, the head statues of Easter Island and the Antikythera mechanism.
• In ancient art throughout the world, themes can be observed which can be interpreted to illustrate astronauts, air and space vehicles, non-human but intelligent creatures, and artifacts of high technology. Däniken also identifies details that are similar in art of unrelated cultures, which he argues imply a common origin.
• The origins of many religions can be described as reactions to contacts of primitive humans with an alien race. In this view, the humans considered the technology of the aliens to be supernatural and the aliens themselves to be gods. According to Däniken, the oral and literal traditions of most religions contain references to visitors from stars and vehicles travelling through air and space. These, he says, should be interpreted as literal descriptions which have changed during the passage of time and become more obscure, rather than symbolic or mythical fiction. One such is Ezekiel's revelation in Old Testament, which Däniken interprets as a detailed description of a landing spacecraft.

Since the publication of Däniken's books, no substantial evidence has been found to verify his claims, and experimenters and historians have made great progress in explaining how structures such as Stonehenge and the Pyramids were built. As a result, most historians regard his claims as pseudoscience or pseudoarchaeology, and are of the opinion that he is drawing far-reaching conclusions from little evidence while disregarding more likely alternative hypotheses. Nonetheless, his theories remain popular with a large group of followers, some of whom have written books of their own.