Alexis Sanchez has completed his move from Arsenal to Manchester United, with Henrikh Mkhitaryan moving in the opposite direction, the English Premier League clubs have announced.
Chile forward Sanchez, who was out of contract at Arsenal at the end of the season, arrives at Old Trafford after three-and-a-half years at the Emirates.
"I am thrilled to be joining the biggest club in the world," Sanchez said on the United website.
"I have spent three-and-a-half wonderful years at Arsenal and I bring with me very positive memories of that great club and its fans.
"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."
United boss Mourinho 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 and a player that makes the team stronger and the supporters proud of their club dimension and prestige."
Sanchez scored 80 goals in 166 appearances for the London club and won the FA Cup in 2015 and 2017, scoring in both finals and was last season's top scorer with 30 goals in all competitions.
Arsenal, meanwhile, announced Mkhitaryan had signed "a long-term contract".
The Armenian, who was 18 months into a four-year deal at United, said: "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."
Arsenal manager Arsene Wenger added: "Henrikh is a very complete player. He creates chances, he defends well, he can absorb distances and he's very committed as well. I must say he's a player who has all the attributes."