The shooting happened at 5:30 p.m. Tuesday, when the Marine fired on his colleague inside a guard shack at Camp Lejeune, Camp Lejeune spokesman Nat Fahy said.
Law enforcement and emergency personnel attempted to revive the shooting victim, who received a single shot, but the person was later pronounced dead at a base hospital, Fahy said.
The shooting came less than a week after a shooting rampage by a soldier at Fort Hood in Texas killed three and wounded 16 others.
Fahy stressed the difference in the two situations.
"We understand that people are at a state of heightened sensitivity, given what happened over at Fort Hood," Fahy said. "It's important that we convey that this is not a Fort Hood-like incident. It was an isolated incident that's no longer active."
The Marine who fired the shot from his M4 rifle was in custody and was awaiting questioning by Naval Criminal Investigative Services, Fahy said Tuesday night.
The name of the shooter, who Fahy said was a male, hasn't been released, and Fahy said the victim's name is being withheld until relatives are notified. There were other guards at the gate, but no one else was hurt.
The shack is about 15 feet by 15 feet and sits under a canopy, Fahy said. He didn't know whether anyone other than the two Marines involved in the shooting were inside the shack at the time.
Fahy said the main gate remained open after the shooting.
He said authorities isolated the area immediately after the shooting, and when it was determined the situation was secure, traffic was allowed to pass on and off the base.
"At no time was the base ever on lockdown," Fahy said.