The 25-year-old forward has three goals and one assist in 24 games this season. He has 33 points (14 goals, 19 assists) in 168 NHL games, all for the Capitals.
Selected by Washington in the third round (No. 77) of the 2012 NHL Draft, Stephenson had seven points (two goals, five assists) and a plus-3 rating during the 2018 Stanley Cup Playoffs, helping the Capitals win the Cup for the first time.
"He's got good NHL experience," Golden Knights general manager Kelly McCrimmon said. "Versatility was a real key for us; he's got the ability to play any of the three forward positions and he's a guy who can move up and down the lineup.
"He's a good penalty killer, a lot of experience in playoff time, obviously, being part of a Stanley Cup playoff team in Washington and just a guy we've had a lot of respect for his game for some time. He's a player that we've talked about in the past and when we had the opportunity to acquire him, we thought it was the right move for us."
Stephenson had no points in five games against the Golden Knights in the Cup Final.
NHL.com Staff Writer Mike G. Morreale contributed to this report
