James van Riemsdyk and Jakub Voracek scored late in the third period to tie the game for the Flyers (29-26-7), who won for the first time in three games. Philadelphia is seven points behind the Carolina Hurricanes for the second wild card into the Stanley Cup Playoffs from the Eastern Conference.
"I think it was the biggest win of the year," Voracek said. "If we lost that game, it would be tough to work from here. ... To be honest, I wasn't believing we were going to win, which is probably bad to say, but ... It's been a good push so far, so we just have to keep it going."
[WATCH: All Penguins vs. Flyers highlights | Complete Stadium Series coverage]
Sidney Crosby had a goal and an assist, Justin Schultz and Evgeni Malkin scored, and Matt Murray made 33 saves for the Penguins (32-22-8), who have lost two in a row. Pittsburgh is tied in points with Carolina for the second wild card.
It was the first win in four outdoor games for the Flyers, who were 0-2-1.
"It was kind of a relief to win an outdoor game, and it was a battle to get it, but sometimes those games ... the hardest to win are the nicest to win," said Giroux, who had an assist on van Riemsdyk's goal. "It didn't look too good, but we found ways to get back in the game and make it 3-3. The experience was great, the fans were so crazy."
Video: EA Sports Overtime Winner: Giroux caps Flyers' rally
Voracek, who had two assists, tied the game 3-3 with 20 seconds remaining after van Riemsdyk scored a power-play goal with 3:04 left to make it 3-2. Each goal came with goalie Brian Elliott (40 saves) pulled for the extra attacker.
Penguins coach Mike Sullivan challenged for goaltender interference by van Riemsdyk on his goal, but it was upheld after video review.
"We're disappointed we didn't get the two points, but I thought our effort was really good," Sullivan said. "We had complete control of the game for the majority of that game with exception of maybe a two-minute span."
Crosby scored off a turnover by Voracek at 7:59 of the first period to make it 1-0. It was his fourth straight game with a goal against the Flyers.
Video: PIT@PHI: Crosby pots nifty backhander to open scoring
Sean Couturier tied it 1-1 at 12:06.
Schultz gave Pittsburgh a 2-1 lead at 10:01 of the second period. His initial shot off a pass from Crosby went wide, but he got to the puck and scored from the right face-off circle.
Malkin made it 3-1 at 6:29 of the third period when his shot hit Elliott's glove, popped in the air, and went in off the back of the goalie's right skate. It was his fifth goal in his past five games.
Video: PIT@PHI: Malkin has shot go in off Elliott's skate
The Penguins played most of the game with four defensemen after Brian Dumoulin and Kris Letang each left because of injury at 16:23 of the first period. Dumoulin has a concussion from a hit by Wayne Simmonds, and Letang was being evaluated for an upper-body injury.
"Those are a tough two guys to replace," Schultz said. "They're two of the more important guys on this team. To lose them early like that wasn't easy. I thought the guys did a good job and played simple. It's unfortunate we couldn't get the win."
Elliott made 30 saves through the second period, including 18 in the first. It was his first start since Nov. 15 because of a lower-body injury, although he played the previous two games in relief.
"It was big in the standings, for sure, against a big rival," he said. "We had a lot of fans in the stands. They braved it, the weather, just as we did out there. It was really fun to play that. ... I definitely didn't want to let the guys down, and they didn't let me down, they came back for me."
The Penguins have lost seven of their past 11 games.
"You've got to move by it, and we've got to learn from it," Crosby said. "I mean, this is kind of a playoff feel, I think, from here on in for us, and sometimes things like this happen."
Video: PIT@PHI: Voracek scores tying goal in waning seconds
They said it
"As we've talked with our players over the last couple of days, we're in a playoff mode right now, and that's just the reality of the League with the parity in the League and the three-point games. There's a lot of teams that are in it, and so we've just got to embrace this challenge." -- Penguins coach Mike Sullivan
"Obviously with four minutes to go in the game, there's a lot of doom and gloom staring us in the face, and our guys didn't change. ... You get two goals with the goalie out ... that doesn't happen too often. A lot of credit to the players and the job they did tonight." -- Flyers coach Scott Gordon
Need to know
Crosby and Malkin have played in each of the Penguins' four outdoor games. ... Van Riemsdyk played his sixth outdoor NHL game (three with the Flyers, three with the Toronto Maple Leafs), tied for most with Patrick Kane, Jonathan Toews, Brent Seabrook and Duncan Keith of the Chicago Blackhawks. ... Penguins center Matt Cullen played his 1,496th game, passing Phil Housley for third among U.S.-born players in NHL history. ... It was the 800th NHL game for Giroux, who scored his 11th overtime goal. ... Pittsburgh forward Patric Hornqvist had an assist for his first point in 15 games.
Penguins: At the Columbus Blue Jackets (7 p.m. ET; ESPN+, FS-O, ATTSN-PT, NHL.TV)
Flyers: Host the Buffalo Sabres on Tuesday (7 p.m. ET; NBCSN, NBCSP, NHL.TV)
Video: Giroux's OT goal caps Flyers' rally in Stadium Series