Pascal Dupuis, Jayson Megna and Kris Letang also scored for Pittsburgh, which snapped San Jose's six-game winning streak. Sidney Crosby added three assists in his 500th career game to push his point total to an NHL-leading 42.
Marc-Andre Fleury made 44 saves for the Penguins against the league's highest-scoring team. Pittsburgh played without star Evgeni Malkin, scratched after sustaining a lower-body injury during the morning skate.
The showdown between two Stanley Cup contenders quickly turned one-sided after a scoreless first period. The Penguins beat Niemi four times in a span of 7:03, showcasing the firepower that makes the Metropolitan Division leaders heavy favourites to make another deep playoff run next spring.
The Sharks have been futilely chasing a trip to the finals for the past decade and they're off to another remarkably hot start. The Pacific Division front-runners lead the league in goal differential and had outscored opponents 24-12 over their last six games.
All that momentum came to a grinding halt during a frantic second period.
Dupuis deflected a slap shot from Brooks Orpik by Niemi just 27 seconds in to give Pittsburgh the lead. Megna made it 2-0 just more than 2 minutes later when his wrist shot from the left circle zipped over Niemi's right shoulder at the end of a 3-on-2 break for his third goal of the season.
Kunitz kept up the barrage at 5:27 when the Sharks turned it over in their own end and the puck ended up on Crosby's stick. Niemi stopped Crosby's wrist shot but Kunitz flipped the rebound into the wide-open net. Kunitz struck again barely 2 minutes later when he buried a loose puck by Niemi for his 16th goal of the season and fifth in his last six games.
The Sharks kept Fleury busy but had trouble finding holes. Hertl made it 4-1 at 9:27 of the second when his centring pass from the bottom of the left circle deflected off a Pittsburgh skate and between Fleury's legs.
San Jose got no closer. Letang beat backup Alex Stalock with a slap shot from just inside the blue line 3:30 into the third to push Pittsburgh's advantage to 5-1 as the Sharks stumbled in the second game of a four-game road trip.
The loss spoiled forward Tyler Kennedy's homecoming. The centre spent six years in Pittsburgh, scoring three game-winning goals during the Penguins' run to the 2009 Stanley Cup and earning minor folk hero status for his perpetually scruffy beard and knack for delivering in big moments.
Kennedy, traded to San Jose last summer, received a loud ovation following a brief video tribute midway through the first period. It was a rare highlight for the Sharks, who were never in it over the final 35 minutes.
NOTES: Dupuis left in the second period and did not return. ... The Penguins are 16-1 when scoring first. ... San Jose D Dan Boyle played in his 900th career game. ... The Sharks scratched F Mike Brown, F Matt Pelech and D Matt Irwin. ... In addition to Malkin, the Penguins also scratched F Zach Sill and D Robert Bortuzzo. ... San Jose plays in Carolina on Friday. Pittsburgh travels to Boston on Saturday.