The clubs reached an agreement for 26-year-old Ziyech, who will remain at the Eredivisie leaders for the rest of the season, earlier this month.

"I am proud to have signed for such a huge club as Chelsea," said Ziyech.

"I am looking forward to next season and hope we can achieve great things."

Ziyech has scored nine goals in all competitions for Ajax this season, contributing to three goals as the Dutch side drew 4-4 against Chelsea in the Champions League in November.

He had been a January transfer target for Chelsea as the Blues suffered a frustrating window and subsequently becomes manager Frank Lampard's first signing after the club's transfer ban.

Chelsea director Marina Granovskaia added: "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."