Liberty is a borough in Allegheny County, Pennsylvania, United States. The population was 2,355 at the 2020 census. Liberty does not have its own post office. The zip code is 15133, and mail is addressed to the city of McKeesport. Liberty is part of the South Allegheny School District, along with Glassport, Lincoln, and Port Vue, and is home to the district's high school and early childhood center.