Cops are calling John Marshall Jones a hero ... not only for rushing out of his house in his underwear to douse a fire, but for chasing down the alleged arsonist behind the blaze.
John, who played special agent Jay Griffin on the Amazon hit "Bosch," tells TMZ ... he was watching TV early Tuesday morning when he looked out the window to see his neighbor's car, front porch and front door on fire.
The veteran actor says he instantly reacted -- bypassing any pants -- and ran out to grab a garden hose to put out the flames. While he was out there, John says a man wearing a ski mask and heavy jacket ran up to the porch and tried to grab something before fleeing.
Once again, Johnny-on-the-spot bolted after the mysterious guy ... who he says reached for a baseball bat. John -- who goes 6'3" and 240 lbs. -- says he wrestled him to the ground, though, and held him until cops arrived.
Law enforcement sources confirmed much of John's account about the 6 AM blaze and his chase of the suspect. We're told John's neighbors were able to escape the burning home, including a teenager and the family dog.
Now, John's being hailed as a true hero for saving lives and nabbing the suspect. Cops claim the guy started the fire with gasoline ... they found a container in his car, and arrested him for arson with great bodily injury.
While you've probably seen John in action in plenty of movies or TV shows ... little did we know he was such a badass off the set?