LAS VEGAS -- The Vegas Golden Knights clinched a Stanley Cup Playoff berth in their inaugural season with a 4-1 win against the Colorado Avalanche at T-Mobile Arena on Monday.

"We've been surprising not only us, but the whole league all season," said Vegas forward Jonathan Marchessault, who had a goal and two assists. "I knew around December we were going to be good, and we just wanted to keep going against the odds."

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The Golden Knights became the first NHL expansion team started from scratch to qualify for the Stanley Cup Playoffs in its inaugural season since 1967-68. The six teams that joined the NHL in 1967 were placed in the West Division, and four qualified for the postseason.

The Golden Knights (48-21-7) lead the San Jose Sharks by six points for first place in the Pacific Division and trail the Nashville Predators by four points for the Western Conference lead.

[WATCH: All Avalanche vs. Golden Knights highlights]

"It's about the next game and getting two more points," coach Gerard Gallant said when asked about the importance of winning the Pacific Division. "The goal is there. We've just got to keep battling. I'm not going to be devastated if that didn't happen, but if that didn't happen, that means we're not playing good hockey and I expect us to play good hockey."

William Karlsson scored his 40th goal and had two assists, and Shea Theodore had a goal and an assist for the Golden Knights, who won for the sixth time in 10 games (6-2-2). Marc-Andre Fleury made 28 saves for his 403rd NHL win, tying Grant Fuhr for 11th in League history.

"I was hoping we could battle for a place in the playoffs," Fleury said. "We've been playing hard all season long and the confidence in our team continues to grow and our expectations are higher. It's nice. It's fun that we did it."

Video: #ThirstForTheCup: Vegas clinches first playoff berth

The Avalanche (41-27-8) lead the St. Louis Blues by one point for the second Western wild card into the playoffs and trail the Los Angeles Kings by one point for the first wild card.

Erik Johnson scored for Colorado, which defeated Vegas 2-1 in a shootout Saturday and lost for the third time in nine games (6-3-0).

"They're all tough when you need the points and don't get them," coach Jared Bednar said. "We got away from our game trying to do too much early."

Alex Tuch ended an eight-game goal drought when he scored on the power play at 4:14 of the second period to give the Golden Knights a 1-0 lead.

Video: COL@VGK: Tuch knocks in loose puck for PPG

Johnson tied it 1-1 at 13:41 when he scored off the rebound of a Blake Comeau shot.

Colorado forward Tyson Jost received a double minor for high-sticking Pierre-Edouard Bellemare at 13:43.

Vegas went ahead 2-1 on Marchessault's power-play goal 33 seconds later, at 14:16, and Theodore scored for the first time since Jan. 2 to make it 3-1 at 1:31 of the third period.

Karlsson scored into an empty net with 16 seconds remaining to make it 4-1.

"Before this season, I didn't think I'd score 40, to be honest," Karlsson said. "I wanted to score more than six goals (his total in 81 games for the Columbus Blue Jackets last season). Safe to say I did that."

Vegas is the third Western team to clinch a playoff berth, joining the Predators and Winnipeg Jets. The Tampa Bay Lightning and Boston Bruins have clinched in the Eastern Conference.

"You look at the NHL, you look at the teams, to make the playoffs is really tough," Gallant said. "For an expansion team to make the playoffs is incredible."

Goal of the game

Theodore's goal at 1:31 of the third period.

Video: COL@VGK: Theodore goes top shelf from the high slot

Saves of the game

Fleury stopping Matt Nieto at 19:08 of the second period and 19:12.

Video: COL@VGK: Fleury keeps the Golden Knights ahead

Highlight of the game

Karlsson's goal at 19:44 of the third period.

Video: COL@VGK: Karlsson finds the empty net for 40th goal

They said it

"I think those were two intense hockey games. It's the way it goes. We managed to get two hard-earned points [Saturday]." -- Avalanche coach Jared Bednar on the home-and-home against the Golden Knights

"It'll be fun. Our fans have been great all season long. It makes it fun to play in this building every night. They'll get to see what playoff hockey is like." -- Golden Knights goaltender Marc-Andre Fleury

Need to know

Avalanche defenseman Tyson Barrie played his 400th NHL game, and forward Gabriel Bourque played his 300th. ... It was Gallant's 200th NHL win. ... Vegas defenseman Deryk Engelland was a healthy scratch for the second game in a row. ... Fleury needs four wins to tie Glenn Hall for 10th in League history (407). ... Marchessault has three goals and four assists in a five-game point streak.

What's next

Avalanche: Host the Philadelphia Flyers on Wednesday (10 p.m. ET; ALT, NBCSP+, NHL.TV)

Golden Knights: Host the Arizona Coyotes on Wednesday (10 p.m. ET; ATTSN-RM, FS-A PLUS, NHL.TV)

Video: Golden Knights beat Avs, clinch playoff spot