The central midfielder, who turned 24 last week, has signed a five-year contract, keeping him at Everton until the end of June 2031. Hackney rose through the ranks at his hometown club before establishing himself as a key player.
He captained Middlesbrough throughout a standout 2025/26 campaign, a season that ended with him being named Championship Player of the Season. Hackney becomes David Moyes' first signing of the 2026 summer transfer window, choosing Everton ahead of a host of other interested clubs.
"I'm delighted to be here.
"As soon as I spoke to the manager, as soon as I knew Everton were interested, to be honest, it was always going to be Everton.
"It's such a big club, with the new stadium and the direction it's going in. I just wanted to be part of that.
"The manager's track record of bringing players from the Championship to the Premier League was a massive part of my decision as well. Hopefully I can be the next one to do it successfully for Everton.
"It feels like a good fit for me, for sure. I think it's a given that you work hard, you run, you make tackles. I'll be doing that.
"I think fans can expect a bit of everything from me — attacking and defensively. I think I can carry the ball well, arrive late in the box, and hopefully score some goals."
Born in Redcar, Hackney is eligible to represent both England and Scotland through his Edinburgh-born mother, having turned out for Scotland at Under-21 level on two occasions.
He was later called up by England Under-21s, making 13 appearances and scoring once for the Young Lions.
Hackney was part of the England squad that won the UEFA European Under-21 Championship in 2025.
"It was big for me to get it done before the start of pre-season. I wanted to be here on day one and do the whole pre-season with the lads. "Obviously, I thank Middlesbrough for the time that I was there, which was unbelievable, but now I feel ready and it's time to make the change."
Everton boss David Moyes said he had been tracking the midfielder for some time.
"Hayden is a promising young player who we've been tracking for some time, and I'm looking forward to working with him.
"We've had a track record over the years of identifying players in the Championship who have gone on to do really well for us and been good investments. We hope that will be the case with Hayden, too.
"He's an England Under-21 international who will provide greater competition in midfield, which is something I wanted going into the new season."