Eminem Shows Off New Beard with Dr. Dre and Kendrick Lamar

Will the real Slim Shady please stand up ... because that dude with the beard standing between Dr. Dre and Kendrick Lamar can't be him, can it?

Eminem posed for a pic with his fellow rap gods and label mates ... along with famed West Coast hip-hop producer FredWreck. It was Wreck's birthday, but Marshall Mathers stole the spotlight with his new facial hair.

The normally baby-faced rapper premiered his scruff at the premiere of "The Defiant Ones" in L.A. Thursday night ... a documentary about the partnership between Dre and Jimmy Iovine.