The Rockcastle River is a 54.8-mile-long (88.2 km) river primarily in Rockcastle County, Kentucky, United States. It is a tributary of the Cumberland River and therefore, via the Ohio River, part of the Mississippi River watershed. In 1750 it was discovered and named the Lawless River by Dr. Thomas Walker and his exploring party. It was soon renamed by hunter Isaac Lindsey for its castle-like rock formations. It is about 60 to 80 feet (18 to 24 m) wide throughout its length. The river has an important native walleye population that has stabilized in recent years. A trip down the lower Rockcastle is one of the most remote trips into the Appalachian wilderness you can have without hiking the Appalachian Trail.