London - Alexis Sanchez said he was "thrilled to be joining the biggest club in the world" as he completed his move to Manchester United from Arsenal on Monday.
The 29-year-old Chile international will reportedly become the highest-paid player in Premier League history following the deal which sees Henrikh Mkhitaryan heading in the opposite direction.
"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," said Sanchez, who joined the Gunners after three years at Barcelona.
"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."
British media have claimed Sanchez will earn a pre-tax weekly salary of around £500,000.
The swap deal that brings an end to Armenia international Mkhitaryan's mixed 18 months at Old Trafford had been expected for several days.
"Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players," said United manager Jose Mourinho.
"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 spent three years at Barcelona, winning La Liga in 2013 before twice lifting the FA Cup with Arsenal but his relationship with the club soured after he tried to orchestrate a move to United's rivals Manchester City in the summer.
He has played 119 times for Chile, whom he helped to win back-to-back Copa America titles in 2015 and 2016, although they failed to qualify for next year's World Cup in Russia.
His move to United was celebrated back in his homeland, where fans on social media lauded the Red Devils' "winning" mentality, in contrast to the paucity of recent success at Arsenal.
"The golden destiny of Alexis," said daily newspaper La Tercera, which claimed the news would warm the hearts of Chilean fans still upset over the country's failure to qualify for the World Cup.
Coach Jose Sulantay told El Mercurio that Sanchez had done well to pick United over Manchester City, rumoured to be his preferred destination until they were outbid by Mourinho's team.
"City never stop passing the ball and that has an impact on the players by masking their individual talent," said Sulantay, the former coach of the Chilean youth teams.
"United are very direct and that's why Alexis's power, technique and speed will be a huge boost for Mourinho's team."The highlight of Mkhitaryan's brief stay in Manchester was scoring in the Europa League final victory over Ajax last season.
He also helped the club win the League Cup, having previously won three Ukrainian titles with Shakhtar Donetsk and four Armenian crowns with Pyunik.
Mkhitaryan joined United for around 30 million in 2016 after three years at Borussia Dortmund.
"I would like to wish Henrikh all the success and happiness that I am sure he is going to get. He is a player that we will not forget, especially for his contribution to our Europa League victory," added Mourinho.
Arsenal also published the news of the swap deal on their website.
"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," said Mkhitaryan.
Arsenal boss Arsene Wenger said he was happy to acquire Mkhitaryan.
"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."
Neither side revealed either financial details of the swap or the duration of contracts each player have signed, although Arsenal described Mkhitaryan's deal as "a long-term contract".