The report will be released to Congress at 11 a.m. ET on Thursday, and then posted publicly on the special counsel's website after that.
Watch this: AG Barr: Trump campaign did not work with Russia in election meddling
Barr opened the press conference with statements about Russian efforts during the 2016 election campaign to spread disinformation on social media, through a company known as the Internet Research Agency, and to hack President Trump's opponents, by Russian military officials connected to the GRU intelligence operation. Among other actions, Barr said, the GRU provided stolen emails and documents to WikiLeaks, which then dumped them into public view.
The attorney general said that while these efforts played a role in the election, Trump's campaign wasn't involved with it.
"The Special Counsel's report did not find any evidence that members of the Trump campaign or anyone associated with the campaign conspired or coordinated with the Russian government in its hacking operations," Barr said.
Watch this: Justice Department indicts 12 Russian cyberspies suspected in DNC hacking
He noted the same for the activities of the Internet Research Agency, the group behind the disinformation that plagued Twitter and Facebook.
"The special counsel found no collusion by any Americans in IRA's illegal activities," Barr said.
Even so, the nearly 400-page report is expected to be a political bombshell. The Justice Department has spent weeks reviewing the material, and Barr said the report would contain "limited redactions," meaning aspects that would be blacked out.
Mueller's team has spent nearly two years investigating President Trump's team and potential links to Russia. Charges were brought against key members including Trump's former attorney Michael Cohen, longtime associate Roger Stone, former campaign chairman Paul Manafort and former National Security Adviser Michael Flynn.
Barr spent much of the press conference noting that Trump's campaign did not coordinate efforts with Russia over these activities. He said "no collusion" four times through the first 20 minutes, and said the report did not find evidence that Trump's team worked with Russian officials on any hacks.
"As you will see, the Special Counsel's report states that his 'investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities,'" Barr said.
Right after Barr ended the press conference, President Trump tweeted a Game of Thrones meme, with "Game Over" emblazoned across a photo of himself.