The Belgian striker elected to move to Goodison Park despite strong interest from West Brom, for whom he scored 17 goals in 35 appearances last season.
The 20-year-old Lukaku tweeted: "Signed @Everton on a season loan :-)"
Lukaku had returned to Stamford Bridge from The Hawthorns at the end of last season, but found his first-team chances had not improved under new boss Jose Mourinho.
Everton were also set to add Wigan midfielder James McCarthy to their squad after reportedly agreeing a fee with the Latics, while a loan move for Manchester City's Gareth Barry was set to be finalised.
More to follow...