Javier Manuel Plata, 24, was taken to a hospital following the shooting, where he later died. Police said the woman was not expected to face charges, although an investigation was still underway.
A neighbor described the neighborhood as mostly "quiet," San Antonio's KENS-TV reported.
"Most of the time people are out here playing in the streets, people riding their bikes around, running, so stuff like that is really surprising to me," Eric Valhalla said.
The unidentified woman was reportedly in the bathroom when she heard Plata walking up the stairs inside her home just before 9.p.m., San Antonio police said.
Officers said she went to grab the weapon and then hid in the bathroom before opening fire when he entered. Plata was shot once in the stomach, the station reported.
Police said the woman had recently broken up with Plata, who was waiting for her when she arrived at the home earlier in the day. They added that she told him to leave and he obliged, but he returned after she had gone upstairs to take a shower, which prompted the shooting.
The woman said Plata had been texting her all day and she previously changed the locks to her home, according to San Antonio's KSAT-TV.
No one else was reportedly in the home at the time.