Justin Bieber got a little to  hands on with Ariana Grande during a performance Wednesday night ... His performance even earned him a tweeter threat from her boyfriend Big Sean, who was in the crowd.

Bieber made a surprise appearance at Grande's concert at the Forum in L.A. Wednesday, where the two performed "As Long As You Love Me." Toward the end of the song, Justin calls her over, tells her to stand in front of him ... and throws his arms around her.

It's pretty apparent ... Ariana's not having it and she quickly disentangles.

But apparently not quick enough for Big Sean, who was in the crowd and saw the whole thing. He tweeted (and later deleted), "This kid is about to learn not to touch my girl like that. Beliebe that."

Bieber himself, oblivious to the ruckus he caused, tweeted after the show, "Love the stage. Thanks for having me @ArianaGrande and thanks LA. Gotta tour again soon :)