The 24-year-old has signed a five-year deal with the option for a further year, and will join the squad for pre-season training in LA.
“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,” the Belgian international told the club’s 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.”
“Romelu is a natural fit for Manchester United. He is a big personality and a big player,” United manager Jose Mourinho said in a statement.
“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.”
Lukaku netted 25 Premier League goals last season for Everton, a tally only bettered by Tottenham’s Harry Kane, who scored 29 times.
The striker, who has also scored 20 goals for Belgium, rejected a new contract at Everton in March and will reunite with United manager Jose Mourinho, under whom he served at Chelsea.
