Johnny Manziel -- 21ST BDAY IN VEGAS ... You Can Find Me In the Club!!

Johnny Manziel turned 21 this morning ... and he rang in his birthday by partying at a Vegas nightclub, TMZ Sports has learned.

The Heisman Trophy winner (and candidate) hit up Hakkasan at the MGM Grand last night (he arrived after midnight) ... and laid pretty low.

He posed for at least one pic with a fan (pictured above) ... but so far, no great tales of drunken insanity.

Shocking, considering his reputation as a party animal. In fact, Manziel once found a loophole in Texas that allowed him to booze at a bar underage (he brought a parent along).

It's not like the Texas A&M quarterback has a football game this weekend ... so no harm, no foul -- at least so far.