Lavaca County (/ləˈvɑːkə/) is a county located in the U.S. state of Texas. As of the 2010 census, its population was 19,263. Its county seat is Hallettsville. The county was created in 1846. It is named for the Lavaca River which curves its way South East through Moulton and Hallettsville before reaching the coast at Matagorda Bay.