Marchessault, who has been with the Golden Knights since their inaugural 2017-18 season, is the first undrafted player to win the award since Hall of Famer Wayne Gretzky in 1988.
The Canadian tied for the NHL lead with 13 goals during the postseason and ranked second with 25 points. All of his goals came over a 13-game span from the second round through the Stanley Cup Final triumph over Florida.
Marchessault was awarded the trophy after a vote by a panel of the Professional Hockey Writers Association, emerging from a cast of contenders that included his teammates Mark Stone and Jack Eichel.
"It is the most proud group I ever had and I'm so happy what we've done this year," said Marchessault. "It's just a surreal feeling, it's a tough trophy (the Stanley Cup) to have.
"That other trophy (the Conn Smythe) is great but to get here and the Stanley Cup you need the whole organization to be great and it could have went to anyone of those guys.
"Just happy to be part of it."
Marchessault, who was not selected in the NHL Entry Draft, signed with the Columbus Blue Jackets as a free agent in 2012. He was traded to Tampa Bay Lightning in 2014 and then spent one season with the Panthers.
But after Florida left Marchessault available in the expansion draft the Golden Knights scooped him up and watched him blossom into a regular threat.