The shooting happened at 6:20pm as the officer was leaving his home with his family, according to Galveston Police.
The officer confronted the woman, who was allegedly holding a gun.
At one point, the off-duty officer shot the woman. She was rushed to UTMB Hospital, where she later died. The officer was not hurt.
The identities of the woman and the officer has not been released.
The Galveston County Sheriff's Office is now investigating, searching for additional witnesses who may have seen the officer-involved shooting.
Deputies interviewed some witnesses shortly after the incident, according to police.
The officer involved has been placed on administrative leave, GPD said.