Terry Crews -- Bieber And Diaz Should Take Beef ... To the Octagon!!!

Terry Crews thinks the war of words between Justin Bieber and Nate Diaz should be a WAR OF FISTS ... saying he firmly believes the best way for the men to settle their beef is to throw down!!!

Crews was out at Craig's when we asked how the guys in Hollywood's newest celebrity feud should settle their beef ... a beef that started when Bieber dissed Diaz's fighting style on social media.

Of course, Diaz fired back on the "TMZ Sports" TV show -- saying, "F**k that fool."

But Terry -- who stars in "Brooklyn Nine-Nine" -- says the time for talk is OVER and it's time for the guys to settle it like men ... IN THE OCTAGON!!!!