New Marshfield is a census-designated place in central Waterloo Township, Athens County, Ohio, United States. As of the 2010 census it had a population of 326. Although it is unincorporated, it has a post office, with the ZIP code of 45766. It is located along State Route 56 west of the county seat of Athens.
The B&O Railroad formerly passed through the community, but the line was abandoned in the 1980s.
A post office called Marshfield was established in 1857, and the post office was renamed New Marshfield in 1909. By the 1880s, (New) Marshfield had a train station and several stores.