Rod Stewart had a rough show in Utah last night ... doubling over in discomfort during the set -- and needing puffs from an oxygen tank onstage.

The singer-songwriter hit the stage at Utah First Credit Union Amphitheatre in West Valley City, UT last night ... and he wasn't moving around the stage as much as usual.

Check out the video, obtained by TMZ ... Stewart's supporting himself on musical instruments, barriers and even a pole at the side of the stage.

He's still tapping his foot, trying to stay with the rhythm ... but clearly there's something wrong -- and several backstage attendants eventually render aid.

One rolls out an oxygen tank, and Stewart takes some big inhales ... steadying himself as the show goes on.

Stewart makes the crowd laugh while also informing them he dang near fainted ... a testament to his good nature -- even in a scary moment. He finished his show sitting in a chair, just needing to get off his feet.

While we don't know the exact reason for Stewart's unsteadiness, West Valley City is about 4,300 feet above sea level ... and the altitude definitely could've hit him harder than he expected. He also recently canceled a show in San Diego, citing laryngitis, though he felt well enough to hit a World Cup match.

We've reached out to Rod for more info ... so far, no word back.