The fire started at about 1 a.m. at ExxonMobil's refinery in Baytown, which is about 25 miles east of Houston.
Four people were injured but everyone else on site has been accounted for, said Rohan Davis, the refinery's manager.
Three of the injured were taken to hospitals by helicopter while the fourth was taken by ambulance, Harris County Sheriff Ed Gonzalez said.
The cause of the fire is not yet known. ExxonMobil said air quality monitoring has not revealed any issues.
No evacuations or shelter-in-place orders have been issued.
The refinery, located along the Houston Ship Channel, has the capacity to process up to 584,000 barrels of crude oil each day. It employs about 7,000 people, according to the company's website.