The 21-year-old was set to be announced at 7pm on Wednesday evening but after all administration issues were cleared up, manager Roberto Martinez and his new acquisition were introduced to the world's media nearly two hours late.
Lukaku has secured a five-year deal with his record fee exceeding the £15m fee paid for his Belgium compatriot Marouane Fellaini in 2008.
Lukaku said: "I am really excited about rejoining Everton. I had a fantastic experience last season and a great relationship with the manager, the chairman, the staff, the players and the fans. I can't wait to get started again and I'm looking forward to a successful season.
"I have a good relationship with the players. This is the place where I belong. I decided quickly I wanted to come back. Here I have the trust of the staff and supporters."
Martinez added: "It’s a very significant day in the history of our Club. Signing Rom is every Evertonian’s wish. This is a footballing statement of what we want to do. Rom will never say it but his target should be to become one of the best world-class strikers.
"We know that Romelu is still a young man and the potential that he has is quite unique, and we are desperate to see him enjoying his football and to watch him grow as footballer in the years to come.
"The arrival of Rom is more than just a signing, it's the fruition of a lot of hard work to get the player we wanted. This is a big moment for Everton's history and the perfect way of getting the squad ready for the start of the season."
Earlier on Wednesday, Lukaku tweeted a photo of himself on an aeroplane with the message: “Time to write a new chapter” and reports grew stronger in the day that the Toffees had finally snared their man with Lukaku not involved in Chelsea's friendly at Vitesse Arnhem and the forward retweeting many goodwill messages.
Lukaku added: "I think I can improve in every aspect. My goalscoring is record is good but I want to get better in every area. I don’t set individual targets. I just want to help my team do better than last season."
Despite the summer departures of strikers Demba Ba and Samuel Eto’o, Lukaku’s first-team chances at Chelsea were bleak following Chelsea’s capture of Diego Costa and the return of Didier Drogba.
The Belgian joined Chelsea from Anderlecht for £18m in August 2011 but his chances were limited, playing just 15 games for the Stamford Bridge club.
The following season he was loaned to West Brom where he scored 17 league goals in 35 appearances, helping the club to eighth spot in the Premier League.
He spent last season on loan at Everton where he became a fans’ favourite at Goodison Park after netting 15 times in 35 Premier League matches.
Everton open their Premier League campaign at promoted Leicester City on August 16.