"We crashed and broke Twitter. We have made history," she told the crowd during the broadcast, when her picture went viral with more than 1.9 million retweets so far and the number rising fast.
She dwarfed the previous record-holder, President Barack Obama, whose post about securing "four more years" in the White House in November 2012 earned 800,000 retweets.
"See, Meryl, what we did, you and I?" she boasted, referring to multiple Oscar-winner Meryl Streep, who crowded in next to a grinning DeGeneres in the photo.
DeGeneres, who joked with A-listers throughout the show, corralled a crowd of stars including Brad Pitt and Jennifer Lawrence.
"You get in here, too, Julia. Lean in," she said to Julia Roberts.
The crowd also included Kevin Spacey, recently of Netflix's "House of Cards" fame, and Lupita Nyong'o, who moments later won an Oscar for best supporting actress for "12 Years a Slave."
"Channing (Tatum), if you can get in also. Bradley (Cooper), come in," the host quipped to loud laughter.
Eventually the group - which also included Jared Leto, Nyong'o's brother and Angelina Jolie - grew big enough that Cooper kneeled in front of DeGeneres and snapped the pic with her smartphone.
"If only Bradley's arm was longer. Best photo ever," DeGeneres posted along with the photograph on her Twitter feed, @TheEllenShow, during a break in her hosting duties.
DeGeneres, who said Twitter officials contacted her to say she broke the record, had her eyes on the prize even before posting the photo, encouraging viewers of the widely-watched show to retweet it and break the record.
Viewers obliged. Within five minutes, it had nearly 100,000 retweets. Less than 90 minutes after that, her tweet had garnered more than 1.9 million retweets and nearly 900,000 "favorites."
"The envelope please... to @TheEllenShow - this is now the most retweeted Tweet with over 1 million RTs. Congrats!" Twitter said on its own feed.