Published: 2019-08-09 04:44:53 BdST
Inter said in a statement on Thursday that Lukaku, who will spearhead the Serie A club's attack in a revamped team under new coach Antonio Conte as they attempt to win their first major trophy since 2011, had signed a contract until June 2024.
No financial details were given but local media said the transfer fee was around 80 million euros ($89.69 million).
"Inter were the only club I wanted," said Lukaku. I’m here to bring the Nerazzurri back to the top."
The powerfully-built left-footed striker was also wanted by Italian champions Juventus, according to media reports.
Television pictures showed several hundred Inter fans waiting to greet Lukaku when he arrived at Malpensa airport on a private jet in the early hours of the morning.
He spent the night at a hotel in the city centre and underwent a medical on Thursday, media reports said.
Still only 26, Lukaku has won 81 caps for Belgium, scored 48 international goals and played at two World Cups.
Inter will be his sixth club in a career which began when he made his debut for Anderlecht as a 16-year-old and has taken him to Chelsea, West Bromwich Albion and Everton. Altogether, he has played 413 competitive club games and scored 187 goals.
Born in Belgium to Congolese parents, his father Roger also played professionally and represented the former Zaire while his brother Jordan plays for Serie A team Lazio.
Lukaku joined United from Everton in 2017. He scored 42 times in 96 appearances in all competitions, with 11 of those goals coming in his first two months at the club. He netted only five league goals for United in the second half of last season.
Lukaku did not play any pre-season matches for United during the close season and spent the last few days training with Anderlecht.
Conte, who replaced Luciano Spalletti at Inter after the end of last season, has been chasing Lukaku for some time and had also tried to sign him when he was manager of Chelsea
Lukaku's arrival will cast further doubt over the future of striker Mauro Icardi, who was stripped of the captaincy last year.
Inter have also signed Austria forward Valentino Lazaro from Hertha Berlin and Italy midfielder Nicolo Barella on loan from Cagliari.
($1 = 0.8912 euros) ($1 = 0.8920 euros)