News that the switch had gone through came via the Toffees' official website.
The move means Klaassen leaves the Dutch giants after 13 years, as he joined the Ajax academy in 2004. The midfielder went on to captain the Amsterdam outfit and won three Eredivisie titles during his time with the team.
Everton will provide the 24-year-old with the opportunity to test himself at a higher level and in a new division. Klaassen has earned that chance.
In the Eredivisie in 2016-17, the midfielder netted an impressive 14 goals and grabbed nine assists, excelling at the hub of a vibrant young team.
Per WhoScored.com, that's a haul no other midfielder in the league could match:
Additionally, the Netherlands international has shown brilliant leadership from the middle of the park, helping Ajax progress into last season's UEFA Europa League final.
As we can see here, he was often the senior figure in a fledgling outfit:
Klaassen was always seeking to take responsibility as a result. On the ball, he's a sound performer, able to distribute intelligently and dribble his way out of tight spaces. But his game, in the main, is about attacking from midfield.
In a midfield trio at Ajax, he was the man most likely to break forward in support of striker Kasper Dolberg and seek to get on the end of chances in the box. Klaassen is a tremendous finisher and composed when shifting the ball in and around the penalty area.
As noted by Patrick Boyland of The Blue Room, it means Klaassen's arrival will most likely need to be supplemented by the Toffees:
The Dutchman may not be a quintessential Ajax graduate in terms of technique and flair, but he has showcased the focus, determination and instincts needed to be a big success elsewhere.
He will not conjure an opportunity out of nothing. Nor is he likely to breeze past a couple of defenders and lash a shot into the top corner. Still, Klaassen has the attributes needed to be a decisive asset for his new club.