Silkosiya (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. Silkosiya 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 catchment area with a territory of 396 ha. The terrain is various, in the lower parts there are predominantly swamp areas with typical Central European flora.