The Canadiens, finally free of a gastro-intestinal virus that raged through their ranks earlier this week, erased a 2-0 deficit with four goals in the final 25 minutes.
Greg de Vries scored in the first period and Brad Larsen scored in the second for Atlanta (24-13-8). But Tomas Plekanec got one back late in the middle frame for Montreal (24-14-5) before Guillaume Latendresse tied the score 51 seconds into the third.
Saku Koivu, who dominated the faceoff circle, won a draw in the Atlanta zone and, two passes later, Ryder scored from the slot for his 12th goal of the season to make it 3-2 at 9:40 of the third.
With 2:47 remaining, Koivu hit Ryder with a pass across the slot for a power-play goal. Koivu earned his 500th career point on the play and got a standing ovation from the sellout crowd of 21,273.
The Thrashers, coming off two overtime losses in which they blew third-period leads, are 1-3-2 in their last six games. Their big three shooters Ilya Kovalchuk, Marian Hossa and Slava Kozlov had no points.
The Canadiens had more jump in their game and outshot Atlanta 11-8 in the first but fell behind 1-0 when De Vries was left alone coming in from the point to bang in Glen Metropolit's pass in at 13:28.
Larsen was left alone to convert Jean-Pierre Vigier's pass 10:36 into the second.
Plekanec got behind defenceman Braydon Coburn to redirect Mike Johnson's pass at 15:02. It was the Canadiens' first even-strength goal in four games.
A turnover at the Atlanta blue-line sent Ryder on a counterattack down the right side to start the third. He fed Latendresse, who came in alone and lifted the puck over Kari Lehtonen.
The Thrashers played the second game of a four-game road trip that takes them to New Jersey on Friday night and Carolina on Saturday. Montreal starts a three-game road series Thursday in Philadelphia, then heads to Ottawa on Saturday and Detroit on Monday.
Notes - Francois St-Laurent was scheduled to officiate the game but was replaced as Bill McCreary's partner by Staphane Auger due to a knee injury. . . Radek Bonk missed a second game with a sore neck while Janne Niinimaa and Aaron Downey were scratched for Montreal. . . Steve McCarthy and Jason Krog didn't play for Atlanta.