After 27 years of service, the Scot has decided that the "time is right" for him to step down.
As you would expect, footballers past and present have been paying tribute to the 71-year-old. Here is what some of them have been saying on the social networking site Twitter.
"Thanks Gaffa for the chance you gave me back in 1992 to play for the BIGGEST club in the world & grace OT as a Man United player #indebted."
Former Man United striker Dion Dublin
"I wish Sir Alex Ferguson an enjoyable retirement. Some bitter memories from many battles, but some oh so sweet too! An amazing manager!"
Former England goalkeeper David James
"Wow sir alex ferguson is retiring?? #endofanera #absolutelegend."
Tottenham Hotspur winger Andros Townsend
"Just woke to see the news on Sir Alex... What he's done at Man Utd has been nothing less than extraordinary! #Legend."
Swansea City winger Wayne Routledge
"Can't believe what I'm reading."
Former Man United defender Ronny Johnsen
"It's just not sinking in! Manchester United with no Sir Alex just doesn't feel right. What a man and great that it has ended on his terms."
Former Man United striker Michael Owen
"Disappointed, shocked, sad. Didn't think THAT day would be today."
Former Man United goalkeeper Peter Schmeichel
"What a career! Proud and honored to have been coached by Sir Alex Ferguson. #thankyougaffer."
Ex Man United striker Giuseppe Rossi
"All at Chelsea would like to wish Sir Alex Ferguson the very best in his retirement. He has been a fantastic adversary over 26 years."
Chelsea Football Club
"An end of an era today! Sir Alex Ferguson, the greatest of all time! United will have a tough time trying to replace him!"
Golfer and Man United fan Rory McIlroy
"Wow S A Ferguson leaving. Hope is health ok. He is the most powerful man i ever met in my life after my dad. Huge respect for Football God."
Former Man United striker Louis Saha
"It is the end of an era for a truly great ambassador, not only for the Premier League but for football overall. Best of luck to Sir Alex."
Former Arsenal and Man City midfielder Patrick Vieira
"2001-2006, 219 games,150 goals under the most successful manager in football history. It was a unique privilege. #SAF."
former Man United striker Ruud van Nistelrooy
"The game just won't be the same without Sir Alex... what a contribution he has made to the game... doubt anyone will ever replicate it."
Celtic manager Neil Lennon
"Thanks for everything, Boss."
Former Man United attacker Cristiano Ronaldo
"26 years, 13 league titles, 2 Champions Leagues, the Cup Winners' Cup, 5 FA Cups, 4 League Cups... Legend!"
Former Man United and current Barcelona defender Gerard Pique
"Sir Alex, one of the best managers of all time. After 26 years of success in the game, we all owe him a tribute."
Man City captain Vincent Kompany