Austin,_Texas | isPartOf | Hays_County,_Texas Hays_County,_Texas | countySeat | San_Marcos,_Texas Austin,_Texas | isPartOf | Williamson_County,_Texas Williamson_County,_Texas | largestCity | Round_Rock,_Texas
Austin is in Hays County and part of Williamson County in Texas. Round Rock is the largest city in Williamson County. The county seat in Hays County is San Marcos.