Rapper Troy Ave is his brother’s keeper … and he proved it, big time, by opening fire on a gunman who ran up on his pal, celeb chef Geoff Cole.

The shootout went down in the San Diego area — home security footage shows Geoff pull up at home in his Bentley, Troy gets out on the passenger side … and as both men are walking toward the garage, a would-be robber sneaks down the alley and charges at Geoff.

The Brooklyn MC immediately reacted, reaching for his own handgun as the gunman fumbled an apparent weapon — and Troy seized the opportunity to start shooting and chasing the guy back into the alley.

It’s unclear from the video if any of Troy’s shots — he squeezed off at least 2 rounds — hit the suspect, but Geoff is giving him props for saving the day. As he put it on IG, “s/o 2 my BROTHER Ave for Holding it Down in a situation where most woulda RAN or FOLDED.”

Although Geoff just posted the video of this incident, we’re told it actually happened back in mid June. He and Troy had been out celebrating the success of Chef Fest … a culinary event held last month in San Diego.

We’ve reached out to reps for Geoff and Troy … as well as cops in San Diego to find out how this ended.