It happened around 5 a.m. in the 2300 Bammelwood Drive near Kuykendahl.
Deputies responded to a shooting call at the apartment when they found a man dead inside from a gunshot wound. The man's girlfriend was still on the scene when they arrived, according to the Harris County Sheriff's Office.
The girlfriend told investigators the man had assaulted her before she pulled out a gun and shot him once. He was pronounced dead at the scene.
No arrests were made and the case will be presented to a grand jury once the investigation is complete, investigators said.