Silkosia (Bulgarian: Силкосия) is a nature reserve in Strandzha Nature Park, located in the homonymous mountain, southeastern Bulgaria. Its territory close to the villages Kosti and Balgari. Silkosia is the oldest reserve in the country, declared on 23 July 1931 in order to protect the evergreen bushes unique for Europe. It encompasses part of the Veleka river catchment area with a territory of 389.6 ha or 3.896 km2. The terrain is various, in the lower parts there are predominantly swamp areas with typical Central European flora. The reserve encompasses territory between 100 and 250 m altitude and is thus among the lowest-lying nature reserves in the country.