Walnutport, Pennsylvania. Walnutport is a borough in Northampton County, Pennsylvania, United States. Incorporated in 1909, Walnutport is located in the Lehigh Valley region of Pennsylvania along the Lehigh River. The population of Walnutport was 2,043 at the 2000 census. The U.S. postal code (zip code) for Walnutport, Pennsylvania, is 18088.