Mongaup Valley is a hamlet (and census-designated place) in the Town of Bethel, Sullivan County, New York, United States. As of the 2020 census, Mongaup Valley had a population of 236. The community is located along New York State Route 17B west of Monticello, and along the Mongaup River. Mongaup Valley has a post office with ZIP code 12762, which opened on June 22, 1848.