Manchester United have signed striker Romelu Lukaku from Everton for an initial £75m on a five-year contract.
The deal for the 24-year-old Belgium international, initially reported by BBC Sport on Thursday, is believed to include £15m in add-ons.
Champions Chelsea had matched the Red Devils' bid, but missed out on re-signing their former player.
"Romelu is a natural fit for Manchester United. He is a big personality and a big player," said boss Jose Mourinho.
"It is only natural that he wants to develop his career at the biggest club.
"He will be a great addition to the group. I am really looking forward to working with him again."
Lukaku, whose contract includes an option for a sixth year, said: "When Manchester United and Jose Mourinho come knocking at the door, it is an opportunity of a lifetime and one that I could not turn down.
"I cannot wait to run out at Old Trafford in front of 75,000 fans."
Speaking to MUTV, he said midfielder Paul Pogba had played a "big role" in him signing for United.
Lukaku sent a farewell message to Everton on Monday in which he thanked the club and said it was "an honour" to play in front of their fans.
- Analysis: Why £75m Lukaku is good move for Man Utd
- Listen: Lukaku will bring different style to United - Phil Neville
Mourinho was manager of Chelsea when they sold Lukaku to Everton for £28m in July 2014.
The striker spent the 2013-14 season on loan at the Merseyside club before signing a five-year deal that summer.
He netted 68 Premier League goals across his two spells, scoring 25 times in the Premier League last season.
Lukaku turned down the most lucrative contract offer in Everton's history, thought to be worth £140,000 a week, in March and later said: "I don't want to stay at the same level. I want to improve and I know where I want to do that."
The Belgian had been on holiday in the United States, and linked up there with United's squad for their pre-season tour.
Lukaku was arrested in Los Angeles last week following a noise complaint and is due is due to appear in court on 2 October.
His arrival at Old Trafford follows Wayne Rooney's departure to rejoin Everton, after 13 years with United.
That deal, which was confirmed on Sunday, was not connected with the Lukaku transfer.
Despite matching United's bid, Chelsea were not willing to match the fees Lukaku's agent Mino Raiola will earn from the move to Old Trafford.
Lukaku in stats
- With two goals against Hull City in March, Lukaku became the first Everton player since Gary Lineker in 1985-86 to score 20 league goals in a season.
- Lukaku is also only the fourth player to score more than 80 goals in the Premier League before turning 24 (after Michael Owen, Robbie Fowler and Rooney).
- He has scored more goals than any other Everton player in Premier League history (68).
- Lukaku is one of only three players to have scored more than 10 goals in each of the past five Premier League seasons (alongside Olivier Giroud and Sergio Aguero).
Top five world record transfers
- £89m - Paul Pogba - Juventus to Manchester United, 2016
- £86m - Gareth Bale - Tottenham to Real Madrid, 2013
- £80m - Cristiano Ronaldo - Manchester United to Real Madrid, 2009
- £75.3m - Gonzalo Higuain - Napoli to Juventus, 2016
- £75m - Luis Suarez - Liverpool to Barcelona, 2014; Romelu Lukaku - Everton to Manchester United, 2017
Top five British transfer records
- £75m - Romelu Lukaku - Everton to Manchester United, 2017
- £50m - Fernando Torres - Liverpool to Chelsea, 2011
- £47.5m - John Stones - Everton to Manchester City, 2016
- £44m - Raheem Sterling - Liverpool to Manchester City, 2015
- £37.1m - Juan Mata - Chelsea to Manchester United, 2014
Mourinho & Lukaku - the story so far
Mourinho on Lukaku expressing surprise he was released by Chelsea to join Everton on loan in 2013:
"Romelu likes to speak. He's a young boy who likes to speak. But the only thing he didn't say is why he went to Everton on loan. That's the only thing he never says. And my last contact with him was to tell him exactly that - 'why do you never say why you are not here?'"
Lukaku on Mourinho in December 2015:
"Everyone says it is Mourinho's fault but it is not his fault because I made the decision [to leave Chelsea]. I went to his office to ask if I could go.
"I signed the deal with Everton at 11 o'clock and the first text message I got on my old BlackBerry was from Mourinho saying 'Good luck, do your best and I will see you next season'."
Mourinho on selling Lukaku to Everton:
"Romelu was always very clear with us with his mentality and his approach was not highly motivated to come to a competitive situation with Chelsea.
"He wanted to play for Chelsea but he clearly wanted to be first-choice striker - but for a club of our dimension it is very difficult to promise to a player."
Find all the latest football transfers on our dedicated page.