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Justin Bieber got a little handsy with Ariana Grande during a performance Wednesday night ... much to the dismay of her boyfriend Big Sean, who went on Twitter and threatened him.

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.