Marchessault fired a wrist shot from the high slot past a screen by William Karlsson to beat St. Louis goalie Jake Allen for the winning goal, his 27th of the season.
Karlsson had a goal and two assists and James Neal also scored for Vegas (49-22-7, 105), which needs just one more point to clinch the Pacific Division title. Malcolm Subban had 21 saves to improve to 12-3-2 this season.
Patrick Berglund, Brayden Schenn and Chris Thorburn scored for St. Louis (43-28-6, 92) which had a season-long six-game win streak snapped. Allen finished with 22 saves for the Blues who fell into a tie with Colorado for the second wild-card spot in the Western Conference, though they have a game at hand.
Berglund, off a nice feed from Steen, gave St. Louis a 1-0 lead just 2:54 into the game when he fired a shot from the slot past Subban's glove side for his 13th goal of the season and fifth in the last nine games.
Vegas rebounded to take a 2-1 lead later in the period when Karlsson scored his 41st of the season on a short-handed breakaway after a long outlet pass from Cody Eakin. Marchessault gave the Golden Knights the lead seven minutes later when, parked at the right side of the net, he slapped in a wide shot by Alex Tuch past Allen's blocker, extending his point streak to seven games, tying his career-high.
Schenn tied it when he took a pass from Jaden Schwartz at the dot in the right circle and beat Subban with a wrist shot that went under his glove for his 26th goal of the season.
Neal scored his first goal since Feb. 8, his 25th, when he roofed a crossing pass from Erik Haula from the right side of the net to give Vegas a 3-2 lead after two periods.
Thorburn then took advantage of a defensive breakdown by the Golden Knights to tie it at the 6:14 mark of the third period, cutting to the net all alone, taking a pass from Scottie Upshall from behind the net and firing a shot through Subban's pads for his first goal since Dec. 11, 2016 against Edmonton.
--Field Level Media
St. Louis | Vegas | |
Patrik Berglund 1 | Points | William Karlsson 3 |
Patrik Berglund 1 | Goals | Jonathan Marchessault 2 |
Kyle Brodziak 1 | Assists | William Karlsson 2 |
N/A | Power Play Goals | N/A |
N/A | Short Handed Goals | William Karlsson 1 |
Jake Allen .846 | Save Percentage | Malcolm Subban .875 |
Jake Allen 22 | Saves | Malcolm Subban 21 |
Team | Shots | Goals | Power Play | Penalty Kill | Penalty Mins | Face Offs Won |
St. Louis | 24 | 3 | 0-2 | 1-1 | 2 | 31 |
Vegas | 26 | 4 | 0-1 | 2-2 | 4 | 21 |