The street artist known as Mr. Brainwash lost more than just his shirt in Vegas ... he lost a big chunk of his identity -- and now he's suing to get it back.
Mr. Brainwash, who's real name is Thierry Guetta, says he's been incorporating the phrase ... "Life is Beautiful" into his street art for years -- and even has trademarks -- but he says a Vegas company has jacked it.
According to the lawsuit -- obtained by TMZ -- a Vegas street fair now goes by "Life is Beautiful" ... and uses an image of a heart made of splashed paint ... which is also one of Mr. Brainwash's trademarks.
Mr. B says the street festival is creating confusion in the street art marketplace. He wants a judge to rip the title and image from the festival.