As well as the Pentagon, the effort is led by the White House, the director of national intelligence, the Energy Department, NASA and the FBI.
The Pentagon said Friday in a post on X that while past administrations sought to discredit or dissuade the American people, President Donald Trump "is focused on providing maximum transparency to the public, who can ultimately make up their own minds about the information contained in these files".
The Pentagon says additional documents will be released on a rolling basis.
Trump, a Republican, has been teasing the announcement since February.
He has previously released records related to the assassinations of President John F Kennedy, senator Robert F Kennedy and Martin Luther King Jr that revealed little beyond what was already known.
The Pentagon has been working on declassifying documents related to UFOs for years, and Congress created an office in 2022 to declassify material.
Its 2024 debut report revealed hundreds of new UAP incidents but found no evidence that the US government had ever confirmed a sighting of alien technology.
Congress ordered the Pentagon to begin releasing decades of files on UFO sightings in 2022 as some members of the military shared encounters with unexplained aircraft.
A small group of Republicans in Congress has pressed for further transparency, accusing the Pentagon of holding documents back.
A March letter from representative Anna Paulina Luna demanded 46 UAP videos identified by whistleblowers.
On social media on Friday, Luna said those videos are expected to be released in a later Pentagon release.
Representative Tim Burchett thanked Trump for "keeping his word" on transparency and disclosure".
"I would like to remind people that transparency won't all happen at once, it will take some time," Burchett said in a statement.
Experts have urged caution around the release of the new files, warning that UAP videos are often misinterpreted and mischaracterised by those unfamiliar with advanced military technology.
The Pentagon's 2024 report rebutted claims that the US government has recovered alien technology or confirmed evidence of alien life.
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