Canadiens goaltender Carey Price appeared to have frozen the puck after stopping a backhander by James Neal, but Neal poked it from under his glove and Letang rapped it into the net in for the game-winner.
Price argued he clearly had his hand over the puck before it was dislodged, but no avail, and the Atlantic Division-leading Penguins improved to 14-6-4.
Marc-Andre Fleury stopped 24-of-27 shots to record the win.
Dennis Rodman pledges to end trips to North Korea
Jessica Simpson shares three-way kiss with friends in photo