Middletown Township is a township in Monmouth County, New Jersey, United States. As of the 2010 United States Census, the township had a total population of 66,522, making it the state's 16th largest municipality, having seen an increase of 195 residents (0.3%) from its population of 66,327 in the 2000 Census, when it was the state's 17th most populous municipality, which had in turn declined by 1,856 (−2.7%) from the 68,183 counted in the 1990 Census. Middletown is one of the oldest sites of European settlement in New Jersey.