A Long Island cop was shot in the back by a cowardly thug yesterday — but his life was spared when his bulletproof vest blocked the round, police said.

The Hempstead Village police officer also took a bullet to his left arm during the 2:30 a.m. confrontation, but still managed to return fire and hit the suspect with three shots, including one to the groin, Nassau Detective Lt. Kevin Smith.

The vest “probably saved the officer’s life,’’ said Smith. “It certainly prevented him from serious injury.”

The trouble began after two plainclothes cops arrived in an area of Hempstead where multiple street robberies were reported.

They attempted to stop two men who fit the description of the robbers for questioning, but one of them fled.

A uniformed officer arrived in a squad car as the man was running off and blocked his path.

The suspect, in his early 20s, ignored the 25-year-old officer’s command to halt and continued to walk through a parking lot near the intersection of Washington Avenue and Cooper Street.

The two-year cop got out of his patrol car and again shouted to the man, who, according to Smith, “starts to take off and reaches into his jacket.”

He fired between two and six shots at the officer, who “pivots and crouches to reach his own weapon,” Smith said.

The cop was struck in the vest in the lower left back and was grazed on the left forearm. Other shots may have narrowly missed the cop.

The cop returned fire while rushing toward the gunman, hitting him three times, including in the groin, said Smith.

The two plainclothes officers rushed over, cuffed the wounded suspect and called for ambulances.

Detectives recovered the suspect’s revolver and a discarded ski mask on the pavement.

The wounded cop was rushed to Mercy Hospital in Rockville Centre, where he was treated and released.

“I would describe the officer’s actions as certainly heroic,’’ said Smith, recalling that, “despite the impact of one round and his wound, he’s still pursuing this guy and returning fire at the same time.’’

The gunman was taken to Nassau University Medical Center in East Meadow. There was no immediate word on his condition, but the injuries were not life-threatening.

The suspect will be charged with attempted murder and other charges, said Smith. Police took the other suspect into custody and were questioning him.