The music mogul has racked up over 150,000 followers in 10 hours and quickly started bragging to judges Paula Abdul, Nicole Scherzinger and LA Reid that he was winning in the popularity poll.
Cowell, tweeting as @SimonCowell, announced: "It's official, I know it's taken a while. It's really me! We're live in 10 minutes. I'll keep you posted. Lots of love #iamsimon. [sic]"
He later taunted: "I love being on twitter. Thank you for allowing me to beat the other 3 in the popularity poll. Paula looked like she'd sucked on a lemon! [sic]"
The 52-year-old X Factor judge joined the social networking site just before Wednesday evening's episode of the US talent show, and posted comments on his acts.
He claimed the female judges were threatened by his act, 14-year-old Drew Ryniewicz's talent.
He tweeted: "Drew was amazing. The girls @PaulaAbdul and @NicoleScherzy are threatened by her."
Cowell also quickly became embroiled in a spat with former America's Got Talent judge Piers Morgan.
Morgan tweeted: "At last! Bring. It. On. love #iampiers. Save your breath @SimonCowell - nobody's watching X Factor, my show's about to start #CNN @PiersTonight."
Cowell hit back: "@piersmorgan look up the word deluded in your dictionary."