Niskanen scored the go-ahead goal in the third period and Sidney Crosby had two assists and Pittsburgh Penguins extended their winning streak to 10 games with a 2-1 victory over the Washington Capitals on Tuesday.
Goaltender Marc-Andre Fleury made 28 saves in his NHL-leading 16th win of the season.
Crosby picked up his league-leading 36th and 37th assists for the Penguins, who were without reigning MVP Evgeni Malkin (shoulder) for the sixth game in a row. The Penguins were also without defenseman Kris Letang (lower body injury).
"Obviously it's an important time in the game and I thought the penalty kill did an amazing job of stepping up to the challenge," Niskanen said.
"We created a lot of energy and some momentum from that. You could feel the crowd getting into it and we were able to go out on a rush there and finish and get the game-winner."
The winning streak is the longest of the season for the Eastern Conference-leading Penguins and the second-longest in the NHL this season. The Penguins are the first team in NHL history to have 10-game win streaks in three consecutive seasons.
"The importance is probably greater just because of a shortened season, but I think the good thing for us is that we're playing so many games and I don't think anyone is thinking a whole lot about it, we're just going out there and playing the way that we need to," Crosby said.
The Penguins have also won a season-high eight games in a row at home.
Braden Holtby made 34 saves for Washington. Alexander Ovechkin scored his 12th goal of the season.
With the score tied 1-1 in the third period, Pittsburgh left wing Matt Cooke received a two-minute penalty for boarding Alex Ovechkin and two minutes for unsportsmanlike conduct at 7:49.
The first two minutes went by without a shot on goal by Washington. Fleury made a glove save on winger Eric Fehr and a kick save on Ovechkin and the Capitals did not get another shot before the power play expired.
"They did a good job keeping us to the outside," Fehr said. "Obviously that was part of the game plan. We just passed it around, maybe trying to get a little too fancy. We've got to find a way to maybe have some more shots and have some traffic. They did a good job boxing out in front. I don't think Fleury was seeing a whole lot of pucks."
The crowd noise in the arena grew as Washington's power play wound down. Niskanen picked off a pass in the crease and got it to Crosby to give the Penguins a 3-on-2 rush into the Capitals end with Cooke joining after he left the penalty box. Cooke fed Niskanen for his fourth goal of the season at 11:58.
"You feel that momentum switch, the place is getting crazy and you need to be able to withstand that," Washington coach Adam Oates said. "But we turned it over at the end. It was a winnable game. At the end of that (power play) there's no way we should give up a turnover. That hurts."
Both teams picked up a power-play goal in the second period. Washington took a 1-0 lead when Ovechkin one-timed a bouncing puck past Fleury for his 12th goal of the season at 8:14.
Pittsburgh tied the game three minutes later when Paul Martin scored his sixth goal of the season, a rising one-timer from the point that sailed into the top right corner of the net. It was the Penguins' seventh shot of the power play and 21st of the game.
"This one's really disappointing," Fehr said. "I thought we had this game. We worked really hard and we did a lot of things we need to do to win games. When you run into a hot team like this they just seem to find a way to win. I thought we deserved that one."
NOTES: Pittsburgh C Evgeni Malkin has missed six consecutive games since taking a hit from Toronto LW James van Riemsdyk on March 9. Malkin hopes to be ready to play Friday at the New York Islanders. ... Pittsburgh D Kris Letang was injured during Sunday's win over Boston. ... The game marked the start of a season-long, four-game road trip for the Capitals. It was also the third and final meeting between the rivals this season. ... Washington D Dmitri Orlov made his season debut. . D Brooks Laich returned after missing 28 games. ... Washington was without defensemen Tom Poti, who was injured Sunday, as well as Mike Green, John Erskine and Tomas Kundratek. ... The Capitals are 4-9-1 on the road with all four wins against opponents from the Southeast Conference.