Beckham, now at Paris St Germain, said Ferguson's protective influence had been vital in his early career.
The former England captain said on his Facebook page: "As I have said many times before the boss wasn't just the greatest and best manager I ever played under he was also a father figure to me from the moment I arrived at the club at the age of 11 until the day I left.
"Without him I would never have achieved what I have done in my career. He understood how important it was to play for your country and he knew how much it meant to me.
"After '98 without the manager I would have found it virtually impossible to cope with the attention I was getting on and off the field and for this I will always be grateful to him for his support and protection.
"I am truly honoured to have been guided by the greatest manager in football and to have had the career that I had under him.
"Thank you boss and enjoy the rest!"
Sir Bobby Charlton hailed Ferguson as "sensational".
Charlton was one of the United directors who appointed Ferguson in 1987 and he said: "He is such a fantastic manager. Everything he has done has been fantastic.
"He is a sensational person in every form and I am really delighted for him.
"I am a director but I hardly do anything because we are winning all the time and it is all down to Sir Alex Ferguson.
"He would get up in the middle of the night and travel 300 miles if he thought there was a school boy that he could sign."
Fellow United old boy Bryan Robson was another surprised by the decision of the man he will always call 'the boss'.
"It definitely came as a surprise," he said. "It would have to happen at some stage and I think the boss must have just felt he has got a lot of grandkids and maybe he wanted to spend a bit more with them."
Red Devils defender Rio Ferdinand also lauded 'boss' Ferguson, tweeting: "The bosses work ethic, his desire to win + to make us better players were unrivalled. Thanks boss."
Former United players also took to Twitter to pay their respects.
Cristiano Ronaldo tweeted a picture of himself and Ferguson on the day he joined Manchester United in 2003, adding: "Thanks for everything, Boss".
Dutch striker Ruud van Nistelrooy also tweeted a picture saying: "2001-2006, 219 games,150 goals under the most successful manager in football history. It was a unique privilege."
Former United keeper Peter Schmeichel described Ferguson's decision to retire as a "bombshell".
"I'm shocked - I just can't make sense of the timing," he told Sky Sports News. "It has come as a bombshell, I really don't know what to make of it.
"Yesterday [Tuesday] I was really happy with what he had done this season, now this, I'm disappointed and very sad.
UEFA president Michel Platini described Ferguson as "a true visionary".
Platini said: "His dedication, his attention to detail and his unique eye for talent, as both the manager of Manchester United FC and Aberdeen FC, has brought rich rewards over a 30-year period.
"He is a true visionary."
The League Managers Association (LMA) added its congratulations.
LMA chairman Howard Wilkinson said: "He is the epitome of the mantra 'Survive, Win, Succeed'. But, in private, with those he trusted, he was the very best sort of friend you could ever wish for. To say his presence on the bench will be sorely missed in no way begins to describe the massive hole he will leave behind. He always said he was too old to retire, let's hope he manages to enjoy the retirement he deserves."
Football Association chairman David Bernstein hailed Ferguson's announcement as "an historic day for football".
He added: "Sir Alex Ferguson's achievements are truly remarkable - he is genuinely one of the greatest managers of all time and certainly of the modern era."
Premier League chief executive Richard Scudamore said Ferguson's "drive, ambition, skill, passion and vision" have forever altered the landscape of football.
Professional Footballers' Association (PFA) chief executive Gordon Taylor, meanwhile, said Ferguson would be "the toughest act to follow".
Taylor told Press Association Sport: "The game of football will be a lot poorer place without him. He has been quite simply the best. He followed in Sir Matt Busby's footsteps and even surpassed him.
"He will be also be the toughest act to follow."
Taylor has been PFA chief throughout Ferguson's 27 years at United and he admitted they had clashed on occasion - but that it was soon forgotten.
He added: "I will miss him - he has been a very good friend of the PFA throughout his career,
"Of course at times it has not always been smooth and we have had a difference of opinion but we always respected each other and we have had a lot more agreements than disagreements."
England rugby coach Stuart Lancaster hailed Ferguson as an example to all young coaches and he believes the Scotsman's his career will never be repeated in any sport.
"What I admire about him most is his longevity and his ability to continually keep his team at the top of the performance cycle," Lancaster said.
"He has always managed to change and renew his team at the right time but keep the energy, the discipline and culture from within and continue to win.
"When you are doing that week in, week out for 60 games a season it's tiring. And when you do that for 20-odd seasons he must be exhausted. I don't think it will be repeated, his career in sport. I can't see how anyone would go and repeat it."
One man who Ferguson has not always seen eye to eye with is Chelsea interim manager Rafael Benitez.
The two have often traded public barbs, but the Spaniard was happy to relent on that on Wednesday night.
"As you know I like to compete against him as a manager but as a person I wish him health and hopefully he can enjoy his retirement and enjoy football in a different way," Benitez said on Sky Sports 1 ahead of Chelsea's clash with Tottenham.
Former United defender Gary Neville, now a pundit, told Sky Sports on Wednesday night: "Once he won that first trophy it has been relentless. Guaranteed success is what he brought to the club. He changed the mentality of the football club from a club that wasn't winning to one that was winning all the time.
"He thinks of people at Manchester United as an island and every one else are sharks - go away.
"Definitely there was a fear factor when you came through the ranks but fear is not a bad thing. At a football club you need control and everyone to be with you. The headmaster being in control of his school.
"During the week there was a very relaxed atmosphere, he let the coaches coach. The coaching staff were never held back by him. On match days he came alive."
Former Liverpool boss Graeme Souness said: "He is handing over a club where every single part of the club is run properly. Arguably the best run club in the world. It would be hard to make a hash of it.
"I think where he has scored enormously is that he has evolved and changed. He was an aggressive manager when he set out but he has mellowed.
"You're inheriting a team that has just won the Premier League by a distance.
"You have to stand back and not just look at the players but at the club and the way it is set up."
Former England manager Glenn Hoddle added: "I don't think it will ever be surpassed. Whoever goes in has a very difficult task.
"My obstacles with him were when I was England manager. He wanted to do what he thought was right for his players. But you have to make tough decisions and put people's noses out of joint."