The 27-year-old Niskanen, the more offensive-minded of the pair, got a $40.25 million, seven-year contract. The 33-year-old Orpik, more of a stay-at-home blueliner who had spent his entire career with the Penguins, was given a $27.5 million, five-year package.
"I've got to be honest: Up until this year, I never really envisioned myself leaving Pittsburgh. As everyone knows, there's been a lot of changes there. It really was a perfect time," Orpik said. "The past year, there was such strong expectations for the team, both internally and externally, and I think we had a really tough time handling those expectations."
The Capitals got off to a fast start on the first day of NHL free agency, also adding Carolina Hurricanes goaltender Justin Peters on a $1.9 million, two-year deal, while re-signing center Michael Latta to a $1.15 million, two-year contract.
Last season, three-time NHL MVP Ovechkin and the Capitals missed the playoffs for the first time in seven years, leading ownership to make changes. George McPhee's contract was not renewed after he spent 17 seasons as the Capitals' GM, while Adam Oates was fired with a year left on his three-season deal.
Brian MacLellan was promoted from assistant general manager to replace McPhee, while Barry Trotz was hired as the new coach.
Washington also has added assistant coach Todd Reirden, who was hired by the Capitals after the Penguins fired him.
Speaking about the Capitals, Orpik said: "I've played against that group enough to know what the potential of that group can be. ... I think they needed some direction, and I think Barry will provide that."
Niskanen, 27, led NHL defensemen with a plus-33 rating last season, when he set career highs with 10 goals and 36 assists. The 6-foot, 209-pound defenseman has 35 goals, 132 assists and 266 penalty minutes in 491 career NHL games for Pittsburgh and Dallas.
The 6-foot-2, 219-pound Orpik has 13 goals, 119 assists and 734 penalty minutes in 703 career games. He is a two-time U.S. Olympian and was an alternate captain for the Penguins since 2008.
"We feel Brooks' leadership and experience will greatly enhance our defense for years to come," MacLellan said. "Brooks plays tough minutes against the opposition's best players."
The 27-year-old Peters went 7-9-4 with a 2.50 goals-against average and a .919 save percentage for the Carolina Hurricanes last season. He also played in six games for Charlotte of the AHL.
Peters made his NHL debut in February 2010, six years after being drafted in the second round by the Hurricanes.
For his career, Peters is 22-31-8 with a 3.05 goals-against average and .904 save percentage.
"When the head coach on the team looks you in the eye and tell you he wants you to be there, that goes a long way," Peters said.
He said that he wasn't told how he'll be used with the Capitals - strictly as a No. 2 behind Braden Holtby, or splitting time in the top goalie spot - but added that he thinks it will "work itself out."
In the Capitals' announcement of the deal, MacLellan is quoted as saying the team believes Peters "is just entering his prime and has a tremendous upside."
Latta, 23, had one goal, three assists and 12 penalty minutes in 17 games for the Capitals last season, when he made his NHL debut.
The Capitals also signed Ds Jon Landry and Mike Moore to one-year, two-way contracts. Both were in the AHL last season.