Authorities say the deputy saw the robbery attempt at a store on Broadway at 6th Street at about 2:30 p.m. and shot one of the suspects.
Two jewelry stores were reportedly the target of the robbers, according to police. The stores are next to each other.
A reserve sheriff's deputy saw one of the suspects was pointing a gun at a security guard inside one of the stores.
"Three suspects got out of apparently two cars, went in simultaneously to hit both these jewelry stores, 601 South Broadway and 603," said LAPD Lt. Paul Vernon.
Police say one suspect in a red hooded sweatshirt went into one of the stores and jumped over the display case and grabbed a fistful of gold chains, then ran out of the store and fled the scene.
Two other suspects armed with guns went into the other store, according to police.
"One of those suspects grabbed the security guard, who was in a uniform, and held him and put a gun to his chest," said Vernon. "That's when an off-duty sheriff's deputy reserve who was in plain clothes saw what was happening from inside the store, pulled his gun and fired at least one time at that suspect, hitting him in the lower back.
"Both suspects came out of the jewelry store," said Vernon. "The one who was shot in the back dropped to the ground or was apprehended by the security officers that work along Broadway here. He dropped his revolver."
Police say the other armed suspect fled the scene. The reserve deputy opened fire on the suspect, but he got away.
Two suspects remained at large Tuesday afternoon.
"Our most recent information is that we have a black Cadillac that's been abandoned west of downtown here," said Vernon. "And apparently initial information is that the tires were shot out of that car."
One of the suspects was considered armed and dangerous.
The wounded suspect who was apprehended received treatment for non-life-threatening wounds.
The investigation was active and ongoing at the scene Tuesday evening.