It happened during the Penguins' 4-2 loss at home against the Blackhawks on Tuesday (their first game of the season) ... when Pittsburgh fans seemingly forgot where they were, and began yelling to get rid of the 51-year-old coach.

One of the fans in attendance captured the moment on video ... and you can clearly hear "Fire Canada" ringing throughout the PPG Paints Arena.

Despite the Steelers' 3-2 record (they currently lead the AFC North) and a win over the Baltimore Ravens on Sunday, the fans are clearly pissed off at the team's offense, and the guy running it.

Canada was hired as the OC in 2021 ... but the offense hasn't exactly been explosive with MC calling the plays, and Kenny Pickett under center.

The Steelers have a bye this week ... probably a good thing for Canada. Regardless of the time off, Pickett told reporters they'll continue to work regardless of the break.

"There's definitely positives, there's always negatives that you can work on during the Bye week," Pickett said.

"There's a lot of things to get better at, too, so I'm gonna use this week to kinda take a step back from it and kinda fine-tooth comb it and figure out what we need to do to get better."

It's not the first time Steelers fans have expressed their frustration with Canada ... earlier this month, a guy brought a sign to an AEW wrestling event, reading, you guessed it ... "Fire Canada!"

They've also put up a billboard around town after raising money via GoFundMe.