"I JUST got a super cool new app called Twitter," wrote the actress, mocking her late arrival to the seven-year-old social media website. "Twelloooo!!!"
"Thank you for the warm welcome!!" she added. "It's so fun and I'm sorry I was so late to the party, now I know why everyone's here!!"
The 41-year-old There's Something About Mary actress added a fun profile picture and a biography in which she describes herself as a "Liver and lover of life; professional laugher, eater, make-believer."
Cameron swiftly followed her fellow Hollywood stars including Gwyneth Paltrow, Drew Barrymore, Reece Witherspoon and Leonardo DiCaprio.
Cameron's fans and famous friends alike Tweeted their excitement to see her presence on the micro blogging site.
"Welcome to Twitter!" wrote chat show host Ellen DeGeneres. "Buenos Diaz."