The 25-year-old center scored an NHL career-high 43 goals (third in the NHL) last season and finished with 78 points in 82 regular-season games. Entering 2017-18, he had scored 18 goals in 183 NHL games and never had more than nine in a season.
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Karlsson, whose arbitration hearing was scheduled for Saturday, can become a restricted free agent July 1, 2019.
"This is a fair deal and I'm glad it's done," Karlsson told the Golden Knights website. "It's a relief and now we can focus on the season ahead.
"... I love playing in Vegas. I couldn't imagine playing anywhere else. I'm hoping we can work out a long-term deal and I'll be here for my whole career."
Selected by Vegas from the Columbus Blue Jackets in the 2017 NHL Expansion Draft, Karlsson led the NHL last season with a plus-49 rating, scored eight power-play goals, four shorthanded goals and six game-winning goals, and won the Lady Byng Trophy, awarded for sportsmanship and gentlemanly conduct.