Plainfield Township is a township in Northampton County, Pennsylvania, United States. The population of Plainfield Township was 6,138 at the 2010 census. The township is part of the Lehigh Valley metropolitan area, which had a population of 861,899 and was the 68th-most populous metropolitan area in the U.S. as of the 2020 census.

Belfast is a census-designated place (CDP) in Plainfield Township in Northampton County, Pennsylvania, United States. It is part of the Lehigh Valley metropolitan area, which had a population of 861,899 and was the 68th-most populous metropolitan area in the U.S. as of the 2020 census. As of the 2010 census, the village's population was 1,257.