J.P. Vigier, Brad Larsen and Steve McCarthy also scored for Atlanta.
Jason Blake scored the only goal for New York, which had a three-game winning streak snapped. Rick DiPietro made 15 saves on a night he struggled in the nets.
The Thrashers finished a stretch in which they played seven of eight on the road. They went 4-1-2 in that stretch and are tied with Buffalo for the Eastern Conference lead. Both teams have 23 points.
Atlanta took a 1-0 lead just 2:59 in when Vigier found a loose puck during a scramble in front and flipped a backhanded shot over DiPietro.
The Thrashers scored twice in a 69-second span late in the period to go ahead 3-0.
Hossa was the beneficiary of a poor clearing attempt by DiPietro. The puck was picked off the left wall by Niko Kapanen, who quickly fired a shot that was tipped in by Hossa at 17:18. His 12th goal moved him into a tie with New York Rangers forward Brendan Shanahan for the NHL lead.
Larsen made it 3-0 at 18:27 by redirecting a shot that DiPietro appeared ready to catch.
Blake got the Islanders on the board with his team-leading eighth goal at 5:13 of the second period. His right circle wrist shot hit the crossbar and bounced off Hedberg’s back and into the net.
Alexei Yashin’s secondary assist on Blake’s goal gave him a team-leading 12 assists and 17 points. Yashin has points in four straight games (two goals, four assists), and six of seven (three goals, 10 assists).
McCarthy scored with 1:02 left in the period on a power play rebound to make it 4-1.
Thrashers forward Scott Mellanby was injured at 7:41 of the third period after he was checked into the boards by Yashin. Mellanby appeared to hurt his left shoulder or arm.
New York lost defenceman Tom Poti at 10:27 when he was inadvertently elbowed in the face.
Notes: After 16 games last season, Atlanta was 6-9-1. … The Thrashers are second in the league in goals scored with 57, trailing Buffalo by two. … Islanders television colour commentator Billy Jaffe worked for the Thrashers on radio and TV the previous six seasons.