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.
The full archive is made up of almost 800,000 files.
They had previously only been accessible at the National Archives in Maryland.
The trove includes the papers of Henry Kissinger, who served as secretary of state under presidents Richard Nixon and Gerald Ford, as well as several hundred thousand pages of intelligence analysis and science research and development.
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, and the recipes for invisible ink.
While much of the information has been technically publicly available since the mid-1990s, it has been very difficult to access.
The records were only available on four physical computers located in the back of a library at the National Archives in Maryland, between 09:00 and 16:30 each day.
At the same time, journalist Mike Best crowd-funded more than $15,000 to visit the archives to print out and then publicly upload the records, one by one, to apply pressure to the CIA.
“By printing out and scanning the documents at CIA expense, I was able to begin making them freely available to the public and to give the agency a financial incentive to simply put the database online,” Best wrote in a blog post.
In November, the CIA announced it would publish the material, and the entire declassified CREST archive is now available on the CIA Library website.
Een nederlandse vertaling is hier te vinden.