The shooting happened outside a Buc-ee's gas station near the Gulf Freeway and Holland Road parking lot at around 10:35 p.m.
Texas City Chief of Police Landis Cravens said the officer was working a second job at the convenience store and attempted to detain a person suspected of shoplifting.
Police said the person in question began resisting the officer, which started a fight between the two.
The off-duty officer then tried other methods, such as a stun gun, to get the person under control before taking out his gun and opening fire, Cravens said.
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The suspect was shot once. They were taken to UTMB Galveston, where they received treatment for non-life-threatening injuries.
The officer was not injured.