The Italy international only moved to Turkey from AS Roma at the start of this year and scored five goals in 10 league games to help Galatasaray wrap up the league title.
But Villa turned their attention towards Zaniolo after Emiliano Buendia suffered a serious knee injury that will rule him out for a major part of the 2023/2024 season.
The 24-year-old is Aston Villa's fourth summer signing after Pau Torres, Youri Tielemans and Moussa Diaby.
The move to Villa Park sees Zaniola reunite with the club's sporting director Monchi, who signed him from Inter Milan for AS Roma in 2018.
“I’m very happy to be here,” he said.
“When Monchi called me, I couldn’t wait to be here and to play my first minutes in this shirt for this amazing club.”
Zaniolo continued: “Aston Villa is one of the best teams in England and in the Premier League. I want to play and show my quality.
“Unai Emery is one of the greatest coaches in the world. And for me, this is a big occasion to show my quality.
“For the team, it’s a chance to do big things.”
“The Premier League is the most important championship in the world.
“It was my dream when I was young that one day I would play in the Premier League.
“Now I’m here and I can’t wait to play.”
Zaniolo has won 13 caps for the Italy national team.
