Kean joined Everton from Juventus last summer for £27m on a five-year contract but failed to impress in his debut season at Goodison Park.
The 20-year-old Italian international made 33 appearances in all competitions last season and scored two goals.
He scored two goals in two Carabao Cup games this season but got limited playing time in the Premier League due to the impressive form of Dominic Calvert-Lewin, whose six top flight goals have propelled Everton to the top of the league table.
Kean is expected to augment a PSG forward line that lost the services of Edinson Cavani and Eric-Maxim Choupo-Moting this summer.
"I am particularly proud and happy to become a Paris Saint-Germain player. They are one of the best clubs in the world as demonstrated by their recent participation in the Champions League final," Kean told PSG's official website.
"I am delighted to join such an ambitious club whose reputation is very important in my country. I can't wait to evolve alongside some of the best players in the world."
PSG are fourth on the Ligue 1 table after six games, having lost their opening two fixtures.
Thomas Tuchel’s side have been drawn alongside Manchester United, RB Leipzig and Istanbul Basaksehir in the group stages of the UEFA Champions League.
