Seth Rogen -- Justin Bieber Is a 'Piece Of Sh*t'

Seth Rogen has spoken ... calling Justin Bieber a "piece of sh*t."

Rogen posted the message on Twitter -- and it immediately blew up, with people retweeting the comment more than 10,000 times.

Rogen and Bieber have actually hung out together in the past -- most notably at the 2011 MTV Video Music Awards, where they seemed like they were getting along in a pretty chummy way.

Apparently, a lot can change in 3 years.