Leo DiCaprio -- Attacked by A-List Wiener Lover

Leonardo DiCaprio's penis now stands proudly next to Will Smith and Bradley Cooper's junk -- because a serial prankster made like a lingerie model and buried his face in Leo's crotch last night.

DiCaprio was arriving at the Santa Barbara Int'l. Film Festival when Ukrainian jokester Vitalii Sediuk ran up, dropped to his knees ... and nuzzled his face against DiCaprio's dong for several seconds.

Leo was stunned, but still managed a smile. He's a pro.

Sediuk is a "journalist" who pulled the same crotch stunt with Bradley last month ... and got slapped by Will when he tried to kiss him at a movie premiere.

Security removed Sediuk from Leo's magic stick, but he was not arrested.