The Nigerian international had a successful loan period with the Turkish club last season.
Coach Vincenzo Montella, who is also a former Adana Demirspor coach, expressed his happiness about the signing.
"Victor has shown great determination to play for Galatasaray, fighting for a transfer from Napoli," Montella said.
"As a young player who is already at the international level, he can set an example for other talented players who want to join Turkish clubs," Montella added.
On his own contract, Montella stated that it was a mutual desire to continue with the club.
"My contract renewal has put an end to a lot of speculation," Montella said.
