Onana leaves Lille after just one season at the club, having joined from Hamburg SV in the summer of 2021.
He made made 42 appearances across all competitions in his only season in France, scoring three goals.
The midfielder has signed a five-year contract at Goodison Park to become Everton's fifth summer signing after James Tarkowski, Ruben Vinagre, Dwight McNeil and Conor Coady.
Onana was in attendance when Everton lost their opening game of the season 1-0 to Chelsea on Saturday.
The transfer to Everton has seen Lille make a considerable amount of profit on the Belgium international, who only joined for €7m.
'It feels great to join Everton. I know it's a big, big club, one of the biggest in England,' Onana told evertontv.
'It's something I want to be part of for many years.
'Everyone here showed they really wanted me and they have a plan for me, so I really appreciated the talks I had with the manager and Director of Football [Kevin Thelwell]. They told me how they wanted me to play and I enjoyed it.
'They both have a lot of ambition and that's the type of guy I am so it matches.
'The manager was one of the big reasons. He played at the highest level, won many things and was a midfielder, too. It means a lot to have interest from him and I think he can teach me many things.
'The Everton fans are amazing. I saw their support last season and it's also one of the reasons why I chose Everton. I am looking forward to meeting them at Goodison and, just hearing about the atmosphere, I get goosebumps.
'Blue is my favourite colour, too! I am going to give everything for the club and our supporters.'
Everton manager Frank Lampard added: Amadou is a player we were really keen to bring to Everton. He has many great qualities to help strengthen our midfield and, at just 20-years-old, has massive potential to get even better.
'I spoke to him about why Everton was the right club for him and we're all looking forward to him showing what he can bring to the team.'
Onana, who has one cap for Belgium, could make his Everton debut against Aston Villa this weekend.
