Chris Thorburn, normally a checking line forward, scored the game-winner 36 seconds into the third period with a seemingly harmless shot from behind the net that banked off goalie Rick DiPietro's left skate for his second goal.
Marc-Andre Fleury made the lead stand up by turning aside 33 of 35 shots, 13 in the final period, as Pittsburgh halted a three-game skid. The Islanders had won three in a row and were 6-1-1 in their previous eight games, but were without leading scorer Alexei Yashin. He is expected to be out two to four weeks with a sprained knee.
Crosby hadn't played since Nov. 20 because of a sore groin after previously missing only one game in his two-season NHL career. He didn't take long to make a difference in a Penguins offence that was limited to five goals in the three losses, making a strong pass in traffic to create Armstrong's first goal 1 1/2 minutes into the game.
Armstrong began the season playing alongside Crosby after scoring 16 goals in 47 games as a rookie last season, but was taken off the top line before Crosby was hurt because of his lack of production.
Armstrong's second goal tied it at 2 midway through the second. Mark Recchi fed the puck across the slot to Crosby in the right circle. Crosby's shot deflected off DiPietro and lay in front of the net before Armstrong put in the rebound.
Viktor Kozlov had answered Armstrong's first goal by scoring his fifth of the season late in the first period, a one-timer from the high slot off Jason Blake's pass from along the boards. Arron Asham put the Islanders up 2-1 by scoring early in the second.
The Islanders missed tying it by an inch with about four minutes remaining, but a sprawling Fleury and Nils Ekman teamed to swat away Andy Hilbert's shot as it sat on the goal line. Several minutes before that, Mike Sillinger's shot deflected off the left post.