Hakim Ziyech has signed a five-year deal with Chelsea ahead of his move to Stamford Bridge this summer.
The Blues announced earlier this month they had agreed a £33.3m deal with Ajax to sign the 26-year-old attacking midfielder at the end of the season, subject to him agreeing personal terms.
Ziyech has now agreed those terms, and told Chelsea's website: "I am delighted and proud to have signed for such a huge club as Chelsea. I am looking forward to next season and hope we can achieve great things together."
He also told Ajax TV that Frank Lampard, and the style of play his Chelsea side are producing, was a big factor in choosing to move to west London.
"It's a big club in a big competition," Ziyech said. "I like the style of play - they play really attacking football. That is something that fits me and that was one of the biggest reasons that I chose Chelsea.
"We [Lampard and I] had a lot of contact in the last couple of weeks. He give me a good feeling."
Chelsea director Marina Granovskaia admitted Ziyech was a "key target" for the Blues in January. The move was not completed last month as Ajax were unwilling to lose an important player mid-season.
Granovskaia told Chelsea's website: "We are delighted Hakim will be joining us in the summer, having been a key target of the club's in this last window.
"He has consistently been one of the most dangerous attacking players in Europe in recent years, which we saw first-hand in our two games against Ajax in the Champions League.
"We wish Hakim and Ajax all the best for the rest of the season and look forward to welcoming him to Stamford Bridge in the summer."
Ziyech has eight goals and 21 assists in 32 games for Ajax this season, helping the club to the top of the Eredivisie.
Van der Kraan: Ziyech will be a hit at Chelsea
Leading Dutch football journalist Marcel van der Kraan insists Ziyech will be hugely missed by Ajax and is no doubt the Moroccan will thrive in a Chelsea shirt.
"He is vital for that team (Ajax). He has been absolutely turning the stadium upside down in certain games," Van der Kraan told Sky Sports News.
"His skills and technique are brilliant. His left foot is fantastic. He has scored cracking goals, [supplied] great assists his figures speaks for themselves.
"I am sure he will carry on doing this at Stamford Bridge."
Why it makes sense for Chelsea to sign Ziyech
Analysis from Sky Sports News reporter Kaveh Solhekol:
Hakim Ziyech to Chelsea makes sense.
Frank Lampard has spoken about signing a long-term replacement for Eden Hazard recently. Ziyech is not as good as Hazard but he is getting there.
He was one of the best players on the pitch when Ajax drew 4-4 at Stamford Bridge in November and he put in two eye-catching performances in the Champions League semi-finals against Tottenham last season.
Ajax are used to selling their best players and it was always likely that Ziyech would leave this summer.
Chelsea have been targeting wide players because Pedro and Willian are out of contract this summer.
Do not rule out Jadon Sancho also moving to Chelsea from Borussia Dortmund in the summer. Lampard has up to £150m to spend but clubs such as Liverpool and Manchester United also want Sancho.
It will now be interesting to see if Chelsea make a move for Ajax goalkeeper Andre Onana. He wants to move to the Premier League and there are question marks over Kepa's future at Stamford Bridge.
Analysis from Sky Sports' Ben Grounds:
Hazard caused most of his damage in Chelsea blue cutting in from his favoured left-sided berth, shifting the ball from left to right on slaloming runs, but it is likely that Ziyech will compete with Callum Hudson-Odoi for a place on the right of the attack.
This is where the Dutch-born Moroccan has been played predominantly by Ajax head coach Erik ten Hag this season and during the club's stunning run to the Champions League semi-finals last term.
Ziyech has been exceptional in this competition for Ajax. This campaign, only Chelsea's Willian (19) created more chances than the 26-year-old's 18. But their failure to reach the knockout stages means they will be in the Europa League later this month.
Last season, Ziyech was the eighth-most creative player in the Champions League, while only Lionel Messi (35) and Neymar (32) completed more dribbles than his 31.
He was one of the best players on the pitch when Ajax drew 4-4 at Stamford Bridge in November - providing two assists - and he put in two eye-catching performances in the Champions League semi-finals against Tottenham last season.
Ajax are used to selling their best players and it was always likely that Ziyech would leave this summer. Their loss is most certainly Chelsea's gain.