The 25-year-old forward will be reunited with former Ibrox team-mate Nikica Jelavic.
Naismsith missed the bulk of last season with a cruciate ligament injury, but has now regained full fitness.
Everton manager David Moyes said: "Steven will be an excellent addition to the players we already have and we look forward to him being fit and ready to play at the start of the season."
Meanwhile, Rangers today said they had reached an agreement with Coventry over the transfer of John Fleck.
His move came amid claims Ibrox chief executive Charles Green would sue players such as Fleck, Naismith, Kyle Lafferty and Steven Whittaker who had walked away from his new regime.
PFA Scotland believes the wantaway players are free agents and have asked the football authorities to release their registration from Rangers.
It was nevertheless understood that any deal taking the 20-year-old midfielder away from the crisis-torn club would be on free-agent terms. But Rangers said a deal had been struck that "recognised Rangers' rights".
"An inaccurate media report this morning suggests the Club is taking legal action against John Fleck over his move to Coventry City.
"Both clubs have in fact reached agreement for the transfer of the player and Coventry have recognised Rangers' rights as part of the process."
