Manchester United appointed Jose Mourinho as manager on Friday on a three-year contract.
Mourinho replaces Louis van Gaal, who was fired on Monday, two days after winning the FA Cup.
The 53-year-old Portuguese had been out of work since being sacked by Chelsea in December, ending his second stint at the club seven months after winning the Premier League title.
Mourinho had been linked with the United job since then and was hired after a whirlwind of negotiations this week.
"To become Manchester United manager is a special honour in the game,” he said in a statement on United's website.
“It is a club known and admired throughout the world. There is a mystique and a romance about it which no other club can match.
"I have always felt an affinity with Old Trafford; it has hosted some important memories for me in my career and I have always enjoyed a rapport with the United fans.
“I’m looking forward to being their manager and enjoying their magnificent support in the coming years.”
United will be Mourinho's sixth club in a managerial career which has seen him win the Champions League twice, with Porto and Inter Milan, as well as capture league titles with Porto, Chelsea, Inter and Real Madrid.
He gives them a manager with the star power to match the hiring of former Bayern Munich and Barcelona boss Pep Guardiola by crosstown rivals Manchester City.
"Jose is quite simply the best manager in the game today,” United executive vice chairman Ed Woodward said.
“He has won trophies and inspired players in countries across Europe and, of course, he knows the Premier League very well, having won three titles here.
“I’d like to take this opportunity to welcome him to Manchester United.
"His track record of success is ideal to take the club forward.”
Mourinho won three Premier League titles in two different spells at Chelsea – 2005, 2006 and 2015.
In between his stints at Stamford Bridge, he managed Inter Milan and Real Madrid with great success, winning the Champions League with the Italian club in 2010.
He also won the Champions League with Porto in 2004, leading to his first hiring at Chelsea and his introduction of himself as "The Special One."
Mourinho returned for his most recent Chelsea spell in 2013 and came third in his first season back.
The Blues won the title by eight points last year and were favorites to lift the trophy again in 2015-16.
But a terrible start, together with a string of off-field problems, left them struggling well down the table and Mourinho was sacked just before Christmas.
Van Gaal finished fourth and fifth in his two seasons and drew criticism from fans and former players for his turgid style of play.
United qualified for the Champions League in his first season but failed to do so in 2015-16 and van Gaal paid with his job, despite winning United's first trophy since Alex Ferguson retired as manager in 2013.