Sebastian is a census-designated place (CDP) in Willacy County, Texas, United States.

[3] According to the United States Census Bureau, the CDP has a total area of 1.7 square miles (4.3 km2), all land.

17.8% of all households were made up of individuals, and 9.5% had someone living alone who was 65 years of age or older.

Sebastian is served by the Lyford Consolidated Independent School District.

The United States Postal Service operates the Sebastian Post Office.