New Plymouth is an unincorporated community in northwestern Brown Township, Vinton County, Ohio, United States. Although it is unincorporated, it has a post office, with the ZIP code of 45654. It is located at the intersection of State Routes 56 and 328 in the county's far northeast.A post office has been in operation at New Plymouth since 1850. New Plymouth was named after Plymouth, Massachusetts, the native home of a large share of the early settlers.