UFC's Robbie Lawler -- Pulls Out of UFC 205

Robbie Lawler has pulled out of his big fight against Donald "Cowboy" Cerrone at UFC 205 in NYC ... TMZ Sports has learned.

Unclear why Lawler backed out of the Nov. 12th fight at Madison Square Garden -- we're working on it.

We're told Cowboy will still fight and the UFC is currently looking for a new opponent.

34-year-old Lawler last fought at UFC 201 back in July -- and lost the UFC welterweight championship belt to Tyron Woodley in a 1st round knockout.