Ferguson, 71, took over from Ron Atkinson in 1986 and won two Champions League crowns, the UEFA Cup Winners' Cup, 13 league titles - including this season's competition - five FA Cups and four League Cups at Old Trafford to make him the most successful manager in British football history following a successful eight years in charge of Aberdeen.
Amid rampant speculation that he would step down in Wednesday's papers, United tweeted: "Sir Alex Ferguson retires. #thankyousiralex".
A club statement read: "The most successful manager in English football history will bow out after the West Bromwich Albion game on 19 May and join the football club board."
“The decision to retire is one that I have thought a great deal about and one that I have not taken lightly. It is the right time,” said Scot Ferguson, who is set for hip surgery in the summer.
"It was important to me to leave an organisation in the strongest possible shape and I believe I have done so.
"The quality of this league winning squad, and the balance of ages within it, bodes well for continued success at the highest level whilst the structure of the youth set-up will ensure that the long-term future of the club remains a bright one.
"Our training facilities are amongst the finest in global sport and our home Old Trafford is rightfully regarded as one of the leading venues in the world.
"Going forward, I am delighted to take on the roles of both director and ambassador for the club. With these activities, along with my many other interests, I am looking forward to the future.
"I must pay tribute to my family, their love and support has been essential. My wife Cathy has been the key figure throughout my career."
Ferguson, who originally retired in 2001 but backtracked on the decision - saying it had been a terrible mistake - won three Scottish titles, a Cup Winners' Cup, four Scottish Cups and a Scottish League Cup with Aberdeen.
The story originally emerged late on Tuesday evening, with reports that United’s player versus coaches golf day at Dunham Massey had been overshadowed by the expectation of such an announcement.
There is speculation that United will announce a successor in the next 48 hours, with David Moyes and Jose Mourinho emerging as favourites. Scot Moyes is out of contract at long-term employers Everton in the summer while Mourinho is widely expected to leave Real Madrid this summer.
The Portuguese had been thought to be set to return to former club Chelsea, but Ferguson vacating the Old Trafford hotseat now throws that anticipated move into doubt.