The new deal will keep the Scotland international at Old Trafford until the summer of 2015.
Fletcher told the club’s official website: “I take great pride in playing for Manchester United, I have won many trophies in my years playing here and I am looking forward to winning many more in the seasons to come.
“I would like to thank everyone at the club.”
United boss Sir Alex Ferguson added: “Darren has been at the club since he was 15 years old.
“I am extremely proud that he came through the ranks of our youth academy.
“I am delighted Darren has signed a new contract, he has developed into a top-class midfielder.”