Rangers sign Hagi on permanent deal
Hagi scored one goal in seven Premiership appearances for the Gers after joining the club in January and was described by manager Steven Gerrard as "a pleasure to coach".
The 21-year-old, who has been capped 10 times by Romania, also featured in three Europa League games during his loan spell at Ibrox, memorably scoring a brace in a 3-2 victory over Braga in February.
Hagi told the club's official website: "I am excited about re-joining the squad for pre-season and putting on the famous shirt once again.
"There was interest from other clubs, but my priority was always to return to Glasgow and play for Rangers.
"I really enjoyed my loan spell at Ibrox and look forward to playing in front of our fans very soon, but for now I hope all of the Rangers fans and your families are staying safe and well."
Hagi, the son of Romania great Gheorghe Hagi, had only moved to Genk from Viitorul Constanta last year.
His next league appearance for Rangers will not come until the 2020-21 season, with 2019-20 having been declared over this month, with Celtic confirmed as champions.