Justin Bieber: Apologizes to Ronda Rousey ... Then Disses Her

It seemed Justin Bieber was finally ready to bury the hatchet with Ronda Rousey -- and then just like that, he reopened the wound.

Bieber sat down with Entertainment Tonight and was asked about Ronda's beef -- which she claims started when he refused to take a picture with her sister at the Cannes Film Festival.

"I guess, supposedly, she asked for a picture with her sister or something, and I guess I said no?" Bieber said ... "But I don’t remember this happening so, Ronda, I’m sorry. Don’t kick my ass!"

Beef over, right?

Wrong ... because moments later, Justin was asked if there was anything he'd like to improve on outside of music -- and he WENT IN on Rousey!

"I would like to get better at boxing ... did you see Ronda Rousey? She got knocked out cold! Oh my God, she got knocked out so bad. Yeah no, she got pummeled."