The shooting happened just after 7 p.m. in a lot on the northeast corner of 23rd Avenue South and South Jackson Street.
Police say two men were arguing in the parking lot when a security guard walked over and asked them to leave. One of the men, thought to be in his 20s, pulled out a gun.
"One of the men involved in the altercation pulled out a handgun and was waving it at the other man he had been fighting with. He then turned and pointed it at the security guard, at which point the security guard fired," said Jeff Kappel of the Seattle Police Department.
The guard fired at least two shots, and the man was hit in the torso. Kappel said it's unclear if the shooting victim fired off any rounds in the altercation.
Medics took the man to Harborview Medical Center with life-threatening injuries.
The other man involved in the altercation ran away before police arrived.
Victor Maposa was in the area Tuesday evening and heard the gunshots.
"Why was a gun being brandished right here?" he said. "Why did a security guard have to shoot somebody else?"
Homicide detectives are investigating the shooting. The security guard was taken to police headquarters to be interviewed.