The Super Eagles forward will turn out for the Lions temporarily next season after a brief stint in the Belgian top-flight that was hampered by injury.
Following his €8 million move to Goodison Park in June 2016, the 21-year-old was loaned out immediately to Anderlecht because of his inability to secure a work permit in the UK.
Onyekuru scored 10 goals in 28 appearances for the Purple and White before suffering a knee injury against Eupen in December that ruled him out for the rest of the 2017-18 campaign.
"Galatasaray are glad to make the signing of Henry Onyekuru official. The 21-year-old winger is signing from Everton on loan," the club tweeted.
The former Eupen star has joined the Lions in their pre-season camp in Switzerland where he will hopes to bond with his new teammates, including Algeria's Sofiane Feghouli and Morocco's Younes Belhanda..