Chelsea have agreed terms with Hakim Ziyech on a five-year contract ahead of his summer move to Stamford Bridge.
The Morocco winger, who will join the Blues on July 1 for an initial fee of 40 million euros ($A65.6 million) is to remain at Ajax for the remainder of the season.
It was announced earlier this month Chelsea had reached a deal with the Dutch club for Ziyech's signature. He is Frank Lampard's first signing as Blues boss.
"I am delighted and proud to have signed for such a huge club as Chelsea," Ziyech told the club's website.
"I am looking forward to next season and hope we can achieve great things together."
Ziyech has laid on more than 20 goals in each of the last three seasons, and has scored 49 times for the Dutch side.
He is viewed as a replacement for Eden Hazard, who left for Real Madrid last summer.