The shooting happened just before 12:30 a.m. at the Valero gas station on the I-69 Eastex Freeway near Greens Road.
Houston police said the security guard saw a man sitting in a car parked at the gas station. He had reportedly been hanging out for a while when the guard approached him and asked how long he would be.
Investigators said a woman arrived and joined the man, and they both became aggressive toward the security guard.
"The security guard then, fearing that it might escalate, he pulls out his handgun and puts it to the side and tells them, 'Hey you guys back up. Y'all just got to leave,'" Lt. Ignacio Izaguirre said.
Police said the man in the car then pulled out a gun of his own and pointed it at the security guard. That's when the guard reportedly shot the man twice in the chest.
The man was pronounced dead at the scene.
Investigators said the security guard and the woman were being questioned about what happened. They were also looking for surveillance video in the area.
It was unclear if charges would be filed.