Bay City is a city and the county seat of Matagorda County, Texas, United States. The population was 17,614 at the 2010 census and 18,061 a…Bay City is a city and the county seat of Matagorda County, Texas, United States. The population was 17,614 at the 2010 census and 18,061 at the 2020 census. According to the United States Census Bureau, the city has a total area of 8.5 square miles, of which 8.5 square miles is land and 0.12% is covered by water.