Millfield is a census-designated place in northern Dover Township, Athens County, Ohio, United States. It had a population of 311 at the 2020 census. It has a post office with the ZIP code 45761. It is located near State Route 13 midway between Chauncey and Jacksonville. Route 13 formerly ran through the community, but a bypass relocated it close by.
A post office was established at Millfield in 1827. Besides the post office, Millfield had a gristmill.
The Millfield Mine disaster, the worst mine disaster in Ohio history, occurred at Millfield in 1930.