There have been seven managers at Chelsea since Mourinho left. He led Chelsea to its first league title in 50 years in 2005 and followed up with another in 2006, and won one FA Cup and two league Cups, but he fell short of giving Abramovich a Champions League trophy, which finally was won with Roberto Di Matteo as coach in 2012.
"I am delighted to welcome Jose back to Chelsea. His continued success, drive and ambition made him the outstanding candidate," Chelsea chief executive Ron Gourlay said. "It is our aim to keep the club moving forward to achieve greater success in the future, and Jose is our No. 1 choice as we believe he is the right manager to do just that."
Chelsea finished third in the Premier League this season and was eliminated in the group stage of the Champions League after winning the title in 2012. Rafa Benitez, hired in November as interim manager after Di Matteo was fired, was repeatedly jeered by Chelsea fans, who distrusted him following his six-year stretch with Liverpool. Benitez led Chelsea to the title of the Europa League, the continent's second-tier tournament, and was hired last week to coach Napoli.
Mourinho's return to Chelsea had been widely expected for months.
"He was and remains a hugely popular figure at the club, and everyone here looks forward to working with him again," Gourlay added.