Dewsbury-Hall has signed a five-year contract with the Toffees with the fee for the 26-year-old an initial 24m Pounds, with a further 3-4m due in add-ons.
He is Everton's fifth signing of the current transfer window following the arrivals of Charly Alcaraz, Thierno Barry, Mark Travers and Adam Aznou.
"I'm honestly very excited, my family's excited. There's a real buzz about this. It feels right for me," said Dewsbury-Hall.
"So just that alone gives me the motivation and the extra determination to show everybody, to prove a point and have a really successful time here.
"The fan base is one of the most passionate in the country, so I know if we start playing well and I start showing the fans what I'm about, it's going to be a big, big bonus to help me and the team show our best. So that's what I want."
Everton manager David Moyes said: "We are pleased that Kiernan has chosen to join Everton. He had other clubs interested in signing him but he was excited by the opportunity and the challenge at Everton.
"Kiernan's quality and experience of the Premier League gives us the hope that he can adapt quickly.
"We look forward to working with him and I'm sure all Evertonians will give him the support he needs."
Everton have opened talks with Manchester City to secure a season-long loan for Jack Grealish, but city are insisting any deal covers the midfielder's hefty wages in full.
Burnley have officially confirmed the signing of French midfielder Lesley Ugochukwu on a five-year contract, continuing their busy transfer activity.
Chelsea are intensifying their pursuit of RB Leipzig's Xavi Simons as they aim to bolster their attacking options for the upcoming Premier League and Champions League campaigns.
Sunderland have announced the signing of highly rated young striker Marc Guiu on a season-long loan from Chelsea for the 2025-26 Premier League campaign.
