The Middle Fork River is a 35.4-mile-long (57.0 km) tributary of the Tygart Valley River in north-central West Virginia, USA. Via the Tygart Valley, Monongahela and Ohio rivers, it is part of the watershed of the Mississippi River, draining an area of 152 square miles (390 km2) on the easternmost part of the unglaciated portion of the Allegheny Plateau. According to the Geographic Names Information System, the Middle Fork River has also been known simply as "Middle Fork" and as "Middlefork."