Manchester United have signed Alexis Sanchez from Arsenal, in a swap deal with midfielder Henrikh Mkhitaryan.
Chile forward Sanchez, 29, who almost joined Manchester City last summer, has signed a four-and-a-half-year deal.
City decided earlier this month that they would not pursue their interest in him as they felt making him their best-paid player may affect team morale.
Mkhitaryan, 29, moves in the opposite direction to Sanchez after scoring 13 goals in 63 appearances for United.
The Armenia international moved to Old Trafford from Borussia Dortmund for £26.3m in July 2016.
???? Ladies and gentlemen, please take your seats. Introducing #Alexis7…#GGMU #MUFC @Alexis_Sanchez pic.twitter.com/t9RIIx4mE4
— Manchester United (@ManUtd) January 22, 2018
The perfect #MondayMotivation! ????
@Alexis_Sanchez #Alexis7 pic.twitter.com/XV1u2VOHvr— Manchester United (@ManUtd) January 22, 2018
He has made only 10 Premier League starts this season, and it is understood there was a breakdown in trust between him and Mourinho.
He has scored 80 goals for the Gunners since joining from Barcelona for £30m in July 2014, and was last season’s top scorer with 30 goals in all competitions.
READ: Alexis Sanchez spotted wearing Manchester United kit for first time
“I am thrilled to be joining the biggest club in the world,” said Sanchez.
“The chance to play in this historic stadium and to work with Jose Mourinho was something I could not turn down.
“I am very proud to be the first Chilean player ever to play for United’s first team, and I hope I can show our fans all around the world why the club wanted to bring me here.”
Sanchez almost teamed up with former Barcelona boss Pep Guardiola in a £60m move last summer, but his deadline-day transfer collapsed when Arsenal failed to sign Monaco midfielder Thomas Lemar.
Since that move did not materialise, Sanchez has had less of an impact, scoring 0.37 goals per game compared to 0.63 last season, with several pundits commenting on his on-field demeanour.
Former Gunners striker Ian Wright said in December that Sanchez looked as though he had “clocked off”.
Arsenal manager Arsene Wenger denied there was a rift between the Chilean and his team-mates, but admitted the uncertainty over his future has affected team morale.
With Sanchez’s contract at Arsenal due to expire at the end of the season – when he would be available on a free transfer – Mourinho has made him his first signing of the January transfer window.
The Portuguese said: “Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players.
“He will bring his ambition, drive and personality – qualities that make a Manchester United player.”
