Texas DPS officials said the deadly wreck happened at about 7:41 a.m. on Interstate Highway 10 east.
The Chambers County Sheriff's Office added that the incident involved an 18-wheeler and the off-duty deputy constable, who was conducting traffic control along the interstate.
The deputy, identified by DPS as 48-year-old Heather Avery, was reportedly in a Ford Explorer with her emergency lights on when a Mack truck driver hit the back of the Ford.
Officials said the truck driver, identified as 30-year-old Murphy Smith, was not injured in the crash.
Chambers County deputies said the off-duty deputy was working for a private contractor at the time.
"Our thoughts and prayers are with the deputy constable's family and loved ones," the sheriff's office added.
An investigation is underway as officials have not said what led up to the crash.