DeGeneres, who joked with A-listers throughout the show, corralled a crowd of stars including Brad Pitt, Meryl Streep and Jennifer Lawrence and posed for a photo that almost instantly circled the globe.
"You get in here, too, Julia. Lean in," she said to Julia Roberts.
The resulting photograph earned nearly a million retweets in minutes in 40 minutes.
The crowd 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 grew big enough that Cooper kneeled in front of DeGeneres and snapped the pic with her smartphone.
Within five minutes, it had nearly 100,000 retweets, and about 45 minutes after that, her tweet it had garnered more than one million retweets.