Fullerton is an unincorporated area and census-designated place in Whitehall Township in Lehigh County, Pennsylvania. The population of Fullerton was 16,588 as of the 2020 census. Fullerton is a suburb of Allentown in 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.