The 18-year-old has agreed to a new four-year deal, with the option of an additional year, after impressing boss Jose Mourinho.
Fosu-Mensah said: “I am delighted to have signed a new contract here at Manchester United. I have enjoyed every minute since I joined this great club and I want to continue the progress that I feel I have made so far. I am learning every day from the manager and the coaching staff. I appreciate the trust that the club has in me and I want to repay that with my performances on the pitch.”
Fosu-Mensah was handed his debut by Louis van Gaal last season and made 10 appearances across all competitions