Jones was doing what has become his ritual before a fight ... he was meditating at a park in Patterson, NJ when he heard a woman scream, "I've been robbed!"
Jones began chasing the man for a good distance. When the fighter caught up with the alleged robber, he knocked the guy's legs out from under him, then placed him in a figure-four leg lock until Paterson cops arrived.
Jones' fight tonight -- UFC 128 -- is in Newark.
Jones tells TMZ, "It was a great pre-fight warm-up for me!"
UPDATE: Jones tweeted about the incident, and even posted a photo (see below) his driver took of the aftermath. He wrote, "I caught him....Coach Jackson finished him ... It feels so good to help others. It gives me power and energy."