Grand Bassa is a county in the west-central portion of Liberia. One of 15 counties that comprise the first-level of administrative division in the nation, it has eight districts. Buchanan serves as the capital with the area of the county measuring 7,936 square kilometres (3,064 sq mi). As of the 2008 Census, it had a population of 224,839, making it the fifth most populous county in Liberia.

The county is bordered by Margibi County to the northwest, Bong County to the north, Nimba County to the east, and River Cess County to the south and east. The western part of Grand Bassa borders the Atlantic Ocean