The Blues were winners for the first time in three tries, getting a backhanded shot from Pietrangelo off a feed from along the left boards by Vladimir Sobotka.
Brian Elliot improved to 8-1-0 for the season with the victory. He made 31 saves.
Marc-Andre Fleury stopped 40-of-43 shots for the Penguins.
In his second game back from the concussion that cost him almost a year of hockey, Pittsburgh's Sidney Crosby went scoreless on two shots. He had two goals and two assists in his return against the New York Islanders.
Notable deaths of 2014 [PHOTOS]
LeBron James reveals his new Nike shoes