The England international scored in an enthralling 3-2 win for the home side, putting the Gunners in-front after Eddie Nketiah had levelled following Marcus Rashford’s opening goal.
Lisandro Martinez later scored having punished Aaron Ramsdale’s mistake and the game looked to be heading for a draw until Nketiah bagged a last-minute winner.
With Manchester City having beaten Wolves earlier in the day, three points against another of their potential title rivals felt huge for Arsenal.
Not only does it keep their lead over the champions to five points with a game in hand, the win also enacts some revenge on the only team to have beaten Arsenal in the League this season.
United conquered Mikel Arteta’s side at Old Trafford in September but could not complete the double, defending in large parts as Arsenal ramped up the pressure.
Speaking after the game, Saka said: “You can see how much it means to everyone. They’re the only team that have beaten us this season so we really wanted to beat them for the fans.
"We know how win this win is for us. It wasn’t easy and it definitely wasn’t our best start to the game but we showed a lot of character to come back and get the win."