Big Sean to Justin Bieber -- Hands Off Ariana ... Or Else!!!

9:45 AM PT -- Big Sean just weighed in again, and tells TMZ ... someone, other than Bieber, pulled a fast one on him.

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.

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

But apparently not quick enough for Big Sean, who was in Canada -- and caught wind of Bieber's antics through social media. He tweeted (and later deleted), "This kid is about to learn not to touch my girl like that. Beliebe that."