The rapper was performing at the Rolling Loud Festival in L.A. Friday and Offset joined him on stage. That's when 21 got the whole crowd to make a pitch to Cardi on behalf of her estranged husband ... and just take him back already.
As we reported ... Offset celebrated his 27th bday at the club later in the evening without Cardi, but wanted to let her know he's still shining brightly.
Earlier in the night, though ... he was begging for his wife to take him back, making it clear his birthday wish is forgiveness and reconciliation.
Not sure the chant from the crowd will help, but we're guessing it won't fall on deaf ears completely.