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.