The defender's existing deal at Old Trafford was due to expire at the end of June.
However, according to the Manchester Evening News, the 20-time English champions have now triggered an extension until the end of next season.
Fosu-Mensah has not turned out for the 20-time English champions since the final day of the 2016-17 Premier League campaign, spending the last two campaigns on loan with Crystal Palace and Fulham.
The Netherlands international has not made a single appearance for Ole Gunnar Solskjaer's side this season as he continues to recover from the knee injury that he picked up while representing Fulham last April.
The 22-year-old has featured twice for the club's Under-23 side this term, though, while he has 21 appearances in total for the Red Devils, including 12 Premier League outings.
The versatile defender joined United from Ajax's Under-17 team in 2014. body check tags ::