The Belgian believes that head coach Unai Emery can take his game to the next level.
Onana is already seen as one of the top young midfielders in Europe at the age of 22, but knows he has to improve to become elite.
“It feels amazing,” he said to club media.
“I’m glad to be here, finally, and I can’t wait to get started.”
On his reasons for joining, Onana added: “It was a bit of everything. There’s a healthy atmosphere here to grow as a player and as a man.
“The team played very attractive football last season with loads of goals and will now be playing in the Champions League.
“I’ll be playing in the Champions League again and that’s very exciting. We also have a manager that’s one of the best in the business and I think he can take my game to the next level.”