EXCLUSIVE

UFC star Nick Diaz sure made everyone THINK he was thrown in jail this weekend when he posted a pic of himself in front of a cop car, but TMZ has learned he was NOT actually arrested.

Diaz posted the pic with the caption "F**k they got us" ... leading many to believe he was referencing a recent prediction he made -- that he will end up in jail because he's never paid taxes in his life.

Seriously, Diaz said ... out loud ... that he's never paid a single cent in taxes -- EVER.

Turns out Diaz did have a run in with cops Sunday night -- but only because he was a passenger in a car that was pulled over for going the wrong way down a one way street.

Sgt. David Moranz from the Citrus Heights PD tells TMZ the driver of the car was arrested on an outstanding warrant and another passenger in the car was arrested for possession of concentrated cannabis ... but Diaz "was never arrested and was released without incidents from the scene."