Seth Rogen is definitely NOT a Belieber!

The 31-year-old appeared on Bravo’s Watch What Happens Live earlier this week and SLAMMED Justin BieberPopdust has video.

"In my opinion, Justin Bieber is a piece of shit,” Rogen told host Andy Cohen. "He seems like he's obnoxious and ungrateful, insincere, and he puts people's lives at danger and overall he acts like a piece of shit."

Whoa! Tell us what you REALLY think Seth!

The actor didn't just talk smack about the Biebs though—he also dished on which celebrities he’s smoked weed with!

And you may be surprised by some of the names!!

What do you think of Seth Rogen's comments about Justin Bieber? Sound off in comments below and tweet us @Popdust