The team said on its Web site Executive Vice President and General Manager Don Waddell would serve as interim coach while the search for a new head coach is carried out.
Hartley was in his fifth season as the Thrashers' coach. He led the team to the NHL Southeast Division title and put Atlanta in the NHL playoffs for the first time in franchise history.
He joined Atlanta in January 2003 after more than four years as head coach of the Colorado Avalanche. He guided Colorado to four consecutive NHL Northwest Division titles and Western Conference finals. The team won the Stanley Cup under him in 2001.
The Thrashers are 0-6-0 this season and have been outscored 9-27. They have been shut out twice, including 4-0 Tuesday by Philadelphia.
Waddell's first game as coach will be Thursday when the Thrashers play at home against the New York Rangers.