The fallen England international is looking to reignite his career after a difficult couple of seasons at White Hart Lane under a series of different managers.
The transfer fee could be as high as £40 million depending on performance-related add-ons.
The 25-year-old is Cup-tied for Saturday’s meeting with Brentford but could make his Everton debut in the Premier League fixture at Newcastle United on Tuesday, 8 February.
Alli told the club’s official website: “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.”
The Lampard factor
Alli’s career has plummeted in recent seasons. Last term, he made only 15 Premier League appearances for Spurs and has clocked up just 10 so far this campaign.
It’s a stark contrast from little more than two-and-a-half years ago when Alli was a key player for Tottenham.
He helped Spurs reach the 2019 Champions League final, while he was a regular in the England team that made it to the World Cup semi-finals the previous summer.
Alli struggled under managers Jose Mourinho, Nuno Espirito Santo and current Spurs chief Antonio Conte, however.
There’s no doubt that sections of Blues fans will understandably feel Lampard is taking a big risk.
But judging by the newly-appointed Everton manager’s previous comments, it’s easy to see why he’s gambling on Alli.
Lampard’s been a huge admirer in the past and will be looking to revive the 25-year-old’s career on Merseyside.
In April 2017, after Alli had bagged his 16th goal of the season in a 4-0 win over Watford, Lampard - who won the Champions League and three Premier League titles during a glittering career - couldn’t hide his admiration.
He even called Alli better than he was at the same age.
“Dele Alli is a special player,” Lampard told Goals on Sunday.
“He is better than me when I was 20, that’s for sure.
“He is like a second striker at the moment but I think that makes sense because the way they play, with the quality in and around the box, he needs to be up there all the time.
“He’s got the knack for runs, he scores great goals like he did yesterday. Seeing goals that he scored in the week against Swansea, he was running in the six-yard box.
“Too many attacking midfield players get towards the penalty spot and then stop and don’t go that extra mile but there’s so many goals to be had. But he does.”
‘A class act’
Alli’s upward trajectory continued the following season - and Lampard continued to be in awe.
In November 2017, Alli, then 21, bagged a double in Tottenham’s memorable 3-1 Champions League victory over Real Madrid - who would go on to win the competition.
Lampard was in the BT Sport studio that evening - and called Alli’s one of the best young players in the world.
He said: “I think I played deeper. I think a while ago now he got moved behind Harry Kane because simply his runs are too good.
“His ability around the box and his knack of getting in the right place was too good and they link so well together.
“This season he’s played up there quite high, then he’s dropped back into midfield. He hasn’t found that consistency of what he found for the second half of last season.
“In terms of young players in world football, his runs and timing and obviously his quality of finishing, is up there with anybody.
“I see those angled runs I tried to make. I always tried to make runs behind defenders - because the defenders were sometimes marking and don’t see that angled run behind.”