Dutchman Henk ten Cate has joined Chelsea as assistant first-team coach to manager Avram Grant.
Ten Cate was assistant at Barcelona before taking over at Ajax
Ajax had already given Chelsea permission to open talks with the 52-year-old Dutchman.
Ten Cate was formerly assistant to Barcelona coach Frank Rijkaard and helped Barca win the Champions League and Spanish title in 2006.
"Chelsea can confirm that Henk ten Cate has joined as assistant first-team coach," said a Blues statement.
Coach Steve Clarke is also in talks to sign a new contract at Chelsea.
An Ajax statement said earlier this week: "Ajax have reached agreement with the English club about the immediate transfer of Henk ten Cate to Chelsea.
"After being approached by Chelsea, Henk ten Cate asked the Ajax management last week for permission to start negotiations with the Premier League side.
"After internal deliberations, Ajax granted permission."
Ten Cate took over at Ajax at the start of last season and led the club to joint top in his first campaign, with the club losing out on the title by goal difference.
Ajax did win two domestic cup competitions but Ten Cate has endured a more difficult time this season.
The club went out of the Champions League in the third qualifying round after being beaten by Slavia Prague and were knocked out of the Uefa Cup by Dinamo Zagreb.
Ten Cate's last game in charge of the Amsterdam outfit was Sunday's 2-2 draw with Sparta Rotterdam, when he and some of the club's hierarchy came in for criticism from some fans.
But Ten Cate defended his Ajax record and said afterwards: "What must I say about that? Under my leadership we have played 41 games, gained 92 points and scored over a hundred goals."