 As "Cossio", it was capital of the ancient tribe of the "Vasates", and under the Romans one of the twelve cities of Novempopulania, when it was known as "Civitas Vasatica"

In later times it was capital of the district of Bazadais, and was the seat of the bishop of the diocese of Bazas from at least the beginning of the 6th century until 1790.And for 250 years prior to 1057, the Bishop of Bazas bore the title of Bishop of Aire, Dax, Bayonne, Oloron and Lescar.According to Gregory of Tours, Bazas had a bishop at the time of the Vandal invasion in the 5th century.