Round Rock is a city in the U.S. state of Texas, in Williamson County (with a small part in Travis County), which is a part of the Greater Austin, Texas metropolitan area. The population was 99,887 at the 2010 census.

The city straddles both sides of the Balcones Escarpment, a fault line in which the areas roughly east of Interstate 35 are flat and characterized by having black, fertile soils of the Blackland Prairie, and the west side of the Escarpment which consists mostly of hilly, karst-like terrain with little topsoil and higher elevations and which is part of the Texas Hill Country. Located about 20 miles (32 km) north of downtown Austin, Round Rock shares a common border with Austin at Texas State Highway 45.