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.