An off-duty NYPD cop waited just one second before ­fatally gunning down another driver in a Brooklyn road-rage incident early Monday.

Exclusive video obtained by The Post shows the moment that Officer Wayne Isaacs fired two shots through his car window at Delrawn Small, who collapsed and died in the street.

The NYPD and the state Attorney General’s Office are investigating the shooting, which the victim’s family said was a clear case of excessive force.

The footage, from Atlantic Avenue and Bradford Street in East New York just after midnight, shows Small, 37, cross two lanes of traffic to reach Isaacs’ car.

He barely has time to look the cop in the eye or even utter a word before Isaacs opens fire, causing him to stagger back.

He stumbles to the ground, gets up for a moment and then collapses again for good.

Isaacs, meanwhile, lurches his car forward a few feet before slamming on the brakes and getting out. He appears to tuck the gun into his waistband as he walks over toward Small.

Isaacs looks in the direction of the dying man, pausing for a few moments near his body, before returning to his vehicle.

He is then seen pacing around and talking on the phone. Sources have said he called 911.

Small’s girlfriend, Zaquanna Albert, 35, then pulls the man’s car across the street before running toward the scene.

Then the footage cuts off.