The St. Louis Blues traded the veteran forward to the Thrashers on Sunday for forward Glen Metropolit, first-and third-round picks in this year's draft and a second-round choice in 2008.
If Atlanta re-signs Tkachuk, the Blues will also acquire a first-round draft choice in 2008.
Tkachuk, who was one of the biggest names on the market heading into Tuesday's 3 p.m. ET trade deadline, is due to become an unrestricted free agent July 1.
The Thrashers paid a hefty price for Tkachuk but knew that Anaheim and Detroit were also in the bidding, especially after the Ducks acquired a first-round pick from Tampa on Saturday in a trade.
But Atlanta faces the pressure of making the playoffs for the first time in its history and avoiding a second-half collapse after a great start. They got things going Saturday night by acquiring veteran defenceman Alexei Zhitnik from Philadelphia.
Heading into Sunday's play, the Thrashers were sixth in the East with 72 points (31-23-10). The Blues, meanwhile, are rebuilding after finishing last overall last season and were 11th in the Western Conference prior to Sunday's action, 12 points out of the final playoff spot with a 26-26-9 record and 61 points.
The Blues may not be finished dealing. Like Tkachuk, veteran forward Bill Guerin will also become an unrestricted free agent on July 1 and is being shopped by St. Louis.
In 61 games this season, the 34-year-old Tkachuk has 21 goals and 23 assists. He has collected 466 goals and 445 assists in 958 career NHL games with St. Louis and Phoenix/Winnipeg.
"We want to thank Keith for his time and dedication to the Blues organization over the past six years," St. Louis president John Davidson said in a statement. "We wish him and his family nothing but the best for the future."
Metropolit has 12 goals and 16 assists in 57 games with Atlanta this season, along with 20 penalty minutes. In 160 career games he has 22 goals and 53 assists while also playing for the Capitals and Lightning.