As part of the deal, Pittsburgh sent defenseman Ian Cole, goalie Filip Gustavsson, a 2018 first-round draft pick and a 2019 third-round selection to the Sens. It also traded forward Ryan Reaves and the Vancouver Canucks' 2018 fourth-round draft pick to Vegas.
To complete the trade, the Pens will receive a 2018 third-round draft pick and forward Vincent Dunn from the Senators, as well as forward Tobias Lindberg from Vegas.
The 30-year-old Brassard has registered 18 goals and 20 assists for 38 points in 58 games this season, and he is just one point away from matching his total from all of 2016-17.
He is in the midst of his 11th NHL season and has four campaigns with 18 or more goals and five of 40 or more points.
Brassard is just two seasons removed from setting a career high with 27 goals for the New York Rangers and three seasons removed from his career high of 60 points.
He is also a quality playoff performer with 55 points in 78 career postseason games, including 11 points during Ottawa's 19-game playoff run last campaign.
To aid in the financials of the deal, the Penguins announced that Vegas will pick up 40 percent of Brassard's remaining salary for the next two seasons.
He is signed through 2018-19.
Pittsburgh lost a quality player in the deal in Cole, as the solid defenseman won the Stanley Cup with the Pens in each of the past two seasons.
He set a career high with 26 points and a plus-26 rating last season, and he has 13 points and a plus-three mark in 47 games for the 2017-18 campaign.
The Golden Knights added some grit to their lineup courtesy of the Penguins, as the 31-year-old Reaves is a big veteran who is willing to drop the gloves. He has four goals and four assists for eight points in 58 games this season.
Pittsburgh is 8-1-1 over its past 10 games, and it has rebounded after a rough start to the season. The Penguins are first in the Eastern Conference's Metropolitan Division with 76 points.
Brassard is capable of playing center or wing, but having him center the third line behind Sidney Crosby and Evgeni Malkin could give the Pens three big-time scoring lines, much like they had before Nick Bonino signed with the Nashville Predators during the offseason.