The records include UFO sightings and psychic experiments from the Stargate programme, which has long been of interest to conspiracy theorists.
The move came after lengthy efforts from freedom of information advocates and a lawsuit against the CIA.
A non-profit freedom of information group, MuckRock, sued the CIA to force it to upload the collection, in a process which took more than two years.
The full archive is made up of almost 800,000 files.
Among the more unusual records are documents from the “Stargate Project”, which dealt with psychic powers and extrasensory perception.
Those include records of testing on celebrity psychic Uri Geller in 1973, when he was already a well-established performer.
Memos detail how Mr Geller was able to partly replicate pictures drawn in another room with varying – but sometimes precise – accuracy, leading the researchers to write that he “demonstrated his paranormal perceptual ability in a convincing and unambiguous manner”.
Other unusual records include a collection of reports on flying saucers, poltergeists and the recipes for invisible ink.
The world is a lot weirder place than you were led to believe.
For anyone who is in a detective mood or wants to dive into ‘wonderland’ here is “STARGATE” and the entire online archive of declassified CIA files!