Nikos Nioplias has left Panathinaikos by mutual consent and been replaced by 71-year-old former Poland boss Jacek Gmoch.
Gmoch has taken over as caretaker coach until a permanent successor has been found.
Nioplias, 45, has been under pressure because of the team’s defensive style of play. He threatened to quit last month but was persuaded to stay on.
“I leave the club as a friend and with the belief I always acted in their best interests,” he said in a statement on Monday.
“We experienced some incredible moments that will remain in my memory including winning the double last year and I fervently wish the team repeat similar successes this year.”
Nioplias replaced Henk ten Cate last December and ended a six-year trophy drought by leading Panathinaikos to the league and cup double.
Panathinaikos drew 1-1 at home to Larissa on Saturday. They have the same points as Olympiakos but are top due to a superior head-to-head record.
Gmoch previously coached the club from 1983-85.
“I am a soldier at the disposal of Panathinaikos … it is not the time for statements but for hard work,” said Gmoch.