The 21-year-old spent last season on loan at Goodison Park, scoring 15 Premier League goals, and has now made his move permanent after not featuring in Blues boss Jose Mourinho's plans for the new campaign.
Everton made a breakthrough in agreeing terms on a deal for the Belgium international - a fee that eclipses the previous €19m club-record signing of Marouane Fellaini.
Lukaku, who enjoyed an extended break after his World Cup exploits with Belgium, was thought to be keen to be reunited with Roberto Martinez, under whom he flourished last term.
And, upon completing his move, he told Everton's official website: "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."
Manager Martinez added: "Welcoming Romelu Lukaku back to Everton on a permanent basis is the news that all Evertonians wanted to hear.
"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."
Chelsea have already moved to overhaul a frontline that misfired last season, with Diego Costa joining from Atletico Madrid and club legend Didier Drogba returning on a one-year deal.