Just hours after the star's new superhero blockbuster The Avengers smashed U.S. box office records by becoming the first movie to break the $200 million (£125 million) mark in its opening weekend, Ruffalo discovered his account on the site had been illegally accessed.
The impostor posted messages about credit card debt and sex tips, and also linked to images of Sportswear Illustrated model Kate Upton and "the best booties" in Hollywood.
The hacker taunted Ruffalo by posting, "I hope you will resolve your passwords. Your account hacked because passwords set very stupid LOL (laugh out loud)."
The drama caused the actor to lose control of his @Mruff221 handle on the site and he is now debating hanging up his old name and taking on a new one.
In a series of posts on the site, he writes, "It's kind of hilarious me getting hacked today. I got to hand it to the hacker. Kind of genius. Okay, I'm back up. The existential battle raged on my keyboard between me and my silly hacker has ended and it seems I lost @Mruff221. to be @mark_ruffalo or to be mruff221."
Updating fans later, he added: "Guess what. I am one and the same!! I am both Identities. @mruff221 and @Mark_Ruffalo. So everyone wins."