ORLANDO BLOOM and JUSTIN BIEBER almost mixed it up at a restaurant in Ibiza, Spain yesterday.

 

Witnesses say it started when Justin PURPOSELY bumped into Orlando.  Orlando then went after him and even THREW A PUNCH, which missed . . . after which Justin yelled, quote, "What's up, [B-word]?"

 

There was a brief confrontation, then Justin left . . . and the crowd applauded.  There's video, but it catches the action AFTER the punch was supposedly thrown. 

 

(Check it out here.)

 

 

 

 

 

 

There were other celebrities in the restaurant, including DIDDY and PARIS HILTON.

 

The bad blood between Orlando and Justin is no doubt FEMALE-INDUCED.  Back in 2012, Justin and Orlando's then-wife MIRANDA KERR hung out backstage at the Victoria's Secret Fashion Show, where Justin performed.

 

And there were rumors that they did more than just hang out.  And earlier this year, Orlando was spotted a few different times hanging out with SELENA GOMEZ.