A speedy left wing, McEachern was acquired from Ottawa in June, but he has played in only 46 games this season, missing the last 21 with groin and back injuries. He had surgery to repair a herniated disk last month.
McEachern, who two weeks ago turned 34, has 10 goals and 16 assists in 46 games this season - his 12th in the NHL. He has 237 goals and 279 assists in 801 NHL contests, also playing for Boston, Los Angeles and Pittsburgh.
The 5-11, 200-pounder was expected to bring a veteran presence to the young Thrashers, who are last in the NHL at 22-35-6-4.
"We acquired Shawn prior to this season because of his experience, veteran leadership and work ethic," General Manager Don Waddell said. "Shawn has delivered in each of those areas on and off the ice for us, and his re-signing strengthens our roster around our core group of players."
McEachern had his best years in Ottawa, totaling 92 goals between the 1998-99 and 2000-01 seasons. He won a Stanley Cup as a rookie with Pittsburgh in 1992.