Sinoe is one of Liberia's 15 counties and it has 17 districts. Greenville is the county's capital. As of the 2022 Census, it had a population of 150,358, making it one of the least populous counties in Liberia. Sinoe has the third-largest area of all Liberia's counties; it has the second least-dense population after Gbarpolu County. The County was originally a colony in the name Mississippi-in-Africa, under auspices of a chapter of the American Colonization Society as it was created with slaves from Mississippi to Liberia.

Sapo National Park is a national park in Sinoe County, Liberia. It is the country's largest protected area of rainforest, was the first national park established in the country, and contains the second-largest area of primary tropical rainforest in West Africa after Taï National Park in neighbouring Côte d'Ivoire. Agriculture, construction, fishing, hunting, human settlement, and logging are prohibited in the park.