The Belgium international signs a five-year deal at Old Trafford off the back of a Premier League season in which he scored 25 goals, narrowly missing out on the Golden Boot.
Lukaku underwent a medical in Los Angeles, were he had been on holiday, and will now link up with his new team-mates who flew out to the United States to begin their pre-season tour on Sunday.
The deal marks a major coup for United, with Lukaku initially appearing destined to return to Stamford Bridge after Chelsea had been working on a deal for months.
But when United’s hunt for a new striker was dealt successive blows – first Antoine Griezmann opted to remain at transfer embargoed Atletico Madrid, then Real Madrid played hard ball over Alvaro Morata – they switched their attention to Lukaku instead.
However, it is believed that the 24-year-old has always been Jose Mourinho’s first choice to succeed Zlatan Ibrahimovic, though the club initially felt his heart was set on a move back to Chelsea.
But talks with Everton over Wayne Rooney – who will move in the opposite direction in a separate deal – opened the door to the possibility of Lukaku joining United.
Rooney moves back to Goodison Park on a free transfer, though his £10m valuation by United is considered to be an unofficial part of the Lukaku deal, while they have also given him a golden handshake, paying off the final year of his contract.
Chelsea, meanwhile, had struggled to reach an agreement with Everton – with the two clubs’ relationship soured during talks for John Stones two years ago – and had been unwilling to pay Mino Raiola’s enormous agent fee.
United, however, enjoy a strong relationship with Raiola, having signed several of his clients over the past 12 months: Zlatan Ibrahimovic, Henrikh Mkhitaryan and Paul Pogba.
Mourinho originally sold Lukaku to Everton while in charge of Chelsea three years, unwilling to cede to his demands to be Chelsea’s first-choice striker, but has been impressed by his development.
Lukaku was delighted to sign on the dotted line for United.
‘I would like to start by thanking Everton and the fans for the last four wonderful seasons, I have made some special friends and we have shared some amazing moments,’ he told the club website.
‘However, 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.
You could see the fight, determination and the spirit in this team during the Europa League final and I want to become a part of that.
‘I cannot wait to run out at Old Trafford in front of 75,000 fans but before that pre-season is where the hard work starts and I am looking forward to that first training session.
Mourinho has broken the bank to get his man and explained why Lukaku is such an excellent addition.
‘Romelu is a natural fit for Manchester United. He is a big personality and a big player,’ he said.
‘It is only natural that he wants to develop his career at the biggest club.
‘He will be a great addition to the group and I know they will make him very welcome. I am really looking forward to working with him again.