An unfortunate, late own goal by David De Gea condemned Manchester United to their eighth defeat of the season and gave Sunderland a huge boost in their fight for Premier League survival.
De Gea and Anthony Martial got in a tangle trying to keep out Lamine Kone's header in the 82nd minute, with the ball going in off the back of the Spain international. The result leaves Louis van Gaal's men six points adrift of fourth place with an extra game played.
But Sunderland's bid to stay up, courtesy of a first Premier League win over United at the Stadium of Light since 1997, saw them climb above Norwich into 18th and just one point off Newcastle and safety.
That two of manager Sam Allardyce's January signings were so influential will be an added positive for the run-in.
Sunderland took a surprise lead after only three minutes through January signing Wahbi Khazri. The Tunisia international scored his first goal for the club after his free kick from the left wing went through everyone and sneaked into the net at the far post.
United offered nothing throughout most of the first half, and Jermain Defoe went close to doubling the lead but stumbled inside in the six-yard box and could only put his shot wide of the post.
Van Gaal suffered another defensive injury blow in the 38th minute when Matteo Darmian suffered a shoulder injury. That meant United had to give a debut to Scotland under-21 international Donald Love -- who had never before been in a matchday squad for the Red Devils and recently returned from a two-month loan spell at League One Wigan Athletic.
But despite struggling to make an impact on the game, the visitors were level in the 39th minute. Vito Mannone got down well low to his right to keep out Juan Mata's effort, but Martial was quickest to the loose ball and scored with a fine lifted finished over goalkeeper from a tight angle.
Sunderland came out of the blocks in the second half desperate to restore their lead. First, they had an appeal for handball in the box by Wayne Rooney waved away and then Defoe came within inches of forcing in a cross at the near post.
Just after the hour, Dame N'Doye, making his first start after signing on loan from Trabzonspor, was put through on goal but De Gea stuck a foot out to make a brilliant save.
Sunderland did get the goal they deserved eight minutes from time. A corner was delivered from the right flank, and another of Sam Allardyce's window signings was left unmarked. Kone, brought in from Ligue 1 side Lorient, headed towards goal and the ball squirmed through De Gea and Martial before eventually going in off the back of the goalkeeper.