Liberty is a city in the U.S. state of Texas, within Houston–The Woodlands–Sugar Land metropolitan area. The population was 8,397 at the 2010 census. It serves as the seat of Liberty County.Liberty is the third oldest city in the state—established in 1831 on the banks of the Trinity River. The city also has a twin of the Liberty Bell from Philadelphia, Pennsylvania. Its area code is 936 and its ZIP code is 77575.