A US marine is free to leave the Northern Territory after being found not guilty of assaulting a Darwin police officer.

Lance Corporal Marquice Bowman was accused of punching the police officer during a brawl outside a Darwin city nightclub last September.

The court heard the fight started when local men targeted a group of American soldiers, throwing bricks and glass bottles at them.

Police officers gave evidence they saw Bowman king hit Constable Joshua Streeter.

But Lance Corporal Bowman said his vision was blurry from pepper spray and when he saw a shape in front of him he thought he was in danger.

The prosecution had told the Darwin Magistrates Court that Bowman made up his "embellished story" and knew he had hit an officer.

The magistrate, however, found him not guilty on all charges, saying Bowman was acting instinctively.