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."
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.
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 is on holiday in the United States and will now link 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.
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