The Merseyside outfit have confirmed on their official website that the England international has signed a contract at the club until the summer of 2024.
The midfielder was struggling for regular game time at the London club, and the move to Everton could help him resurrect his career.
A few years ago, the 25-year-old was regarded as one of the finest prospects in English football, but his development has stalled, and he needed a fresh start to kick start his career.
On deadline day, Everton has also secured a loan move for the Manchester United midfielder, Donny van de Beek.
The Toffees recently appointed Frank Lampard as the club’s new manager, and it will be interesting to see whether the former Chelsea boss can get the best out of his new signings over the next few months.
Alli will be hoping to establish himself as a regular starter for the Merseyside outfit now.
If the 25-year-old manages to recapture his confidence and form, he could be a key player for Everton going forward.
The Toffees are missing goals and creativity from their midfield, and the former Tottenham midfielder will add just that.
On joining Everton, Dele said: “I’m delighted to have signed for Everton, a huge club with a great fanbase and history. I’m eager to get started and can’t wait for my first game in an Everton shirt. I’m looking forward to helping the team and the opportunity to work with the new manager Frank Lampard.”