Chatham Borough, New Jersey. Chatham is a borough in Morris County, New Jersey, United States. As of the 2010 United States Census, Chatham's population was 8,962, reflecting an increase of 502 (+5.9%) from the 8,460 counted in the 2000 Census, which had in turn increased by 453 (+5.7%) from the 8,007 counted in the 1990 Census.