The 26-year-old Moroccan international will become Frank Lampard’s first signing since becoming Chelsea manager last summer if the deal goes through.
Chelsea were under a two-window transfer ban when Lampard took over, and although that restriction was successfully overturned following an appeal, the Blues were unable to sign any players during the January transfer window.
Ziyech has impressed for Ajax since joining the club from FC Twente in the summer of 2016.
He was particularly outstanding last season as Ajax won the Eredivisie title, KNVB cup and reached the semi-finals of the UEFA Champions League.
Ziyech has already faced and impressed against Chelsea this season in the UEFA Champions League, playing the full 90 minutes in a 1-0 defeat at the Johan Cruijff Arena in October before supplying three assists when the two clubs drew 4-4 in the reverse fixture at Stamford Bridge in December.
In 29 matches in all competitions for Ajax this season, the former Heerenveen man has scored nine goals and provided 12 assists in the Dutch top flight.
Ziyech has represented the Moroccan national teams 32 times, scoring 14 goals.
He was a member of the Atlas Lions’ squad that played at the 2018 FIFA World Cup in Russia.
