The 37-year-old midfielder has gone straight into the squad for today's FA Cup trip to Manchester City.
"It's fantastic that Paul has made this decision. It's always sad to see great players end their careers, but especially so when they do it early," said United boss Sir Alex Ferguson.
"But he has kept himself in great shape and I always felt that he had another season in him. It's terrific to have him back."
Former England international Scholes added: "I've been pretty clear since I stopped playing that I miss it.
"I'm delighted the manager feels I can still make a contribution to the team and I'm looking forward to playing my part in trying to bring more success to this great club."
Scholes retired at the end of last season.