Justin Bieber was getting snapped and then he snapped ... unloading on a paparazzi for saying something the pop star felt was disrespectful.

Ya gotta see the heated interaction between Justin and a photog ... with the Biebs snapping at the guy, set off by the dude saying "Thank you."

Justin asks the photog -- who is not a TMZ cameraman -- why he's thanking him ... and explains why it's disrespectful to do so.

Bieber was heading back to his SUV with Hailey Bieber after grabbing breakfast at Beverly Glen Deli in Los Angeles ... and paps were waiting for him.

Justin told the guy to stick to taking pictures and letting him do his thing ... with no need to thank JB for anything.

We don't usually see this kind of reaction from Justin ... so it's pretty interesting.

Justin and Hailey hanging out in public is noteworthy too ... the couple was dogged by break-up rumors last month but they squashed it and now they're out together.