Alexis Sanchez has completed his move from Arsenal to Manchester United in a swap deal for Henrikh Mkhitaryan, the Premier League clubs announced on Monday.
The deal ends a lengthy standoff between Sanchez, who joined the London club from Barcelona in 2014, and Arsenal, who were determined not to lose him on a free transfer when his contract runs out this summer.
"I am thrilled to be joining the biggest club in the world," Sanchez said in a statement on United's website.
He added: "The chance to play in this historic stadium and to work with [United manager] 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 had been heavily courted by Manchester City, who were close to a £55 million deal with the forward last August before Arsenal ended negotiations.
Those talks rekindled this month, only for City, 12 points clear of United at the top of the Premier League, to decide earlier this week to end their pursuit out of fear his addition would disrupt the balance of the squad.
Instead, the 29-year-old will head to City's rivals United, who were the only club willing to move for a player entering the final six months of his contract.
Sanchez has scored seven Premier League goals and eight in all competitions for Arsenal this season and finished with 24 goals in the league, and 30 overall in 2016-17.
"Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players," Mourinho said.
"He will bring his ambition, drive and personality, qualities that make a Manchester United player and a player that makes the team stronger and the supporters proud of their club dimension and prestige."
Mkhitaryan, who turned 29 on Sunday, was among Mourinho's first signings a United manager but quickly fell out of favour. Arsenal said he had signed a "long-term contract."
United reportedly paid £26 million to sign him from Borussia Dortmund in the summer of 2016, but Mkhitaryan only started one Premier League game before December as he adjusted to England.
He eventually worked his way into the squad and made 63 appearances in all competitions in one and a half seasons with United, scoring 13 times, including a goal in the Europa League final.
But Mourinho said in November that he was unhappy with how the Armenia international was "disappearing" during games, and Mkhitaryan and his manager were then involved in a row during a video analysis session at the club's Carrington training ground, sources told ESPN FC.
Mkhitaryan said in an Arsenal statement: "I'm very happy that we could finish this deal and I'm very happy to be here. It's a dream come true because I've always dreamed of playing for Arsenal. Now that I'm here, I'll do my best for this club to create history."
Though no transfer fee was mentioned in the deal, Arsenal are likely to enjoy some relief from Sanchez's wages, which unconfirmed reports have placed at as high as half a million pounds per week.