Round Rock is a city in Williamson County, Texas, United States. The population was 99,887 at the 2010 census. Round Rock is the 31st most populous city in Texas and the 4th most populous city in the Greater Austin metropolitan area. It is the fastest growing large city in the United States and the second most populous capital city after Phoenix, Arizona. As of the 2016 U.S. Census estimate, Round Rock had a population of 121,786. Located about 20 miles (32 km) north of downtown Austin, Round Rock shares a common border with Austin at the latter city's southern limit.
Round Rock was the site of the first permanent settlement in the Williamson Valley, which is now part of Greater Austin. The city was founded in 1851 and incorporated in 1854. The city has been a hub of economic activity since the late 19th century, when it became a major stop on the Chisholm Trail cattle route and later the Austin and Northwestern