The defenseman gave the Blues a 3-2 lead with a backhand from the bottom of the right hash marks at 16:34 after Logan Couture scored twice in 1:59 earlier in the period for the Sharks. It was Bortuzzo's first Stanley Cup Playoff goal in 31 games.
"It's not really my game, but it's definitely something that goes in the back of your mind," said Bortuzzo, who has 14 goals in 335 NHL regular-season games. "... It was just a rush of emotion."
[WATCH: All Blues vs. Sharks Game 2 highlights | Complete series coverage]
The best-of-7 series is tied 1-1 with Game 3 at Enterprise Center on Wednesday (8 p.m. ET; NBCSN, CBC, SN360, TVAS).
Jordan Binnington made 24 saves for the Blues, who improved to 6-2 on the road in the playoffs.
Video: STL@SJS, Gm2: Binnington denies Meier with mask, arm
Martin Jones made 21 saves for the Sharks, who lost Game 2 after winning Game 1 for the third straight series.
"Every series we get a great first game and the second kind of a little bit struggling," San Jose forward Tomas Hertl said. "It's still fine, but we have to be better, we can't play like that."
Couture tied the game 2-2 at 6:54 of the second with a wrist shot from the left face-off circle after he took an outlet pass from Timo Meier. It was his NHL-leading 13th goal.
Couture scored unassisted on a shorthanded breakaway at 4:55 to get the Sharks to within 2-1. He deflected Alex Pietrangelo's shot near the San Jose blue line, and then outraced the St. Louis defenseman before beating Binnington with a backhand from the top of the crease.
Video: STL@SJS, Gm2: Couture pots SHG after forcing turnover
"I thought Logan Couture was pretty good, a couple other guys," Sharks coach Peter DeBoer said. "But it's tough to win this time of year if you don't have everybody going, and I thought they got contributions from everybody like we did [in Game 1]. So we've been here before, and we know how to handle this."
Bortuzzo, Pietrangelo and Blues defensemen Vince Dunn and Joel Edmundson combined for five points (two goals, three assists). St. Louis defensemen have 38 points (seven goals, 31 assists) in the playoffs, tied with San Jose for the NHL lead.
Video: Clutch Performance: Bortuzzo gets first playoffs goal
"We want our D to be active, and they've produced for us all year and they're producing for us now in the playoffs too," Blues coach Craig Berube said.
Bortuzzo blocked a point-blank shot by Kevin Labanc at 9:56 of the third period to help the Blues maintain their one-goal lead.
"He's a gamer. Absolutely," St. Louis forward Brayden Schenn said. "He's a warrior that no matter the circumstances, in or out, he's a team guy, a locker room guy, a glue guy that you need on your team. When he has a big game like that tonight, you got to be pumped for him."
Oskar Sundqvist scored at 16:52 of the third with a one-touch backhand off a pass from Alexander Steen from the bottom of the right circle for the 4-2 final.
Video: STL@SJS, Gm2: Steen connects with Sundqvist for tally
Jaden Schwartz gave St. Louis a 1-0 lead 2:34 into the game when he scored from the slot with a wrist shot over Jones' glove for his Blues-leading ninth goal.
"It's definitely the funnest time of the year," said Schwartz, who scored 11 goals in 69 regular-season games. "I had some struggles earlier that I haven't really had before, but once playoff starts ... I don't know, you just embrace the games, have fun with it. I don't really try to think too much about it."
Dunn scored with a wrist shot through traffic at 4:16 of the second period to make it 2-0.
Binnington is 4-2 with a .932 save percentage in the playoffs following a loss.
"It starts with our goalie," forward Ryan O'Reilly said. "I think [Binnington] was unbelievable tonight. Made some huge saves. He's leading the charge that way."
They said it
"In my opinion, that wasn't anywhere near what we're capable of, and I don't think we've played up to our capabilities in a while now. ... And it's discouraging, it's frustrating, because we're going to need everyone here if we're going to beat these guys, because they're a very good hockey team." -- Sharks forward Logan Couture
Video: STL@SJS, Gm2: Couture nets second goal for milestone
"Obviously a good start to the hockey game for us. We're a group that sticks together, don't get too down on each other." -- Blues forward Brayden Schenn
"We're fine. We've got a good group in here. We've been resilient all year. This isn't going to deter us by any means." -- Sharks center Joe Thornton
Need to know
Pietrangelo tied the Blues record for points by a defenseman in one postseason with 12 (two goals, 10 assists) with an assist on Sundqvist's goal (Joe Micheletti, 1981; Jeff Brown, 1990 and 1991; Al MacInnis, 1999). ... St. Louis has outscored opponents 13-3 on the road in the third period in the playoffs, including 11-2 in six wins. ... Couture's second goal was his 100th playoff point with the Sharks (47 goals, 53 assists). He trails Patrick Marleau (120 points) and Thornton (112) in San Jose history. … Couture is one goal from tying Joe Pavelski's Sharks record for most goals in one postseason. Pavelski scored 14 in 2016, when San Jose lost in six games to the Pittsburgh Penguins in the Stanley Cup Final. … The Blues were 0-for-5 on the power play and are 1-for-26 in their past eight games.
Game 3 of Western Conference Final at Enterprise Center on Wednesday (8 p.m. ET; NBCSN, CBC, SN360, TVAS)
Video: Blues win Game 2, even series with Sharks