Freshwater ecosystems have a greater value for biodiversity per surface area than terrestrial and marine ecosystems. Streams and lakes are notably prone to biodiversity loss, being the greatest threats habitat destructions and nonnative species. The aim of the present study is to assess the fish composition of lotic and lentic water systems of an island ecosystem (Gokceada Island, Turkey), which is poorly studied, and to highlight the entry routes and possible impacts of translocated fish species in an island ecosystem. The field surveys were conducted in a total of 14 sampling sites (five reservoirs and nine streams) in the inland waters of Gokceada Island between September 2019 and January 2021. Fish samples were collected by electrofishing (SAMUS 1000; a backpack electro-shocker). A total of eight fish species belonging to six families (Anguillidae, Atherinidae, Cyprinidae, Leuciscidae, Mugilidae, and Poeciliidae) were determined. As a result fish community of the island was mostly composed by the introduced species Carassius gibelio, Gambusia holbrooki and Pseudorasbora parva. This study provides the first detailed information on the distribution of the fish species encountered in the inland waters of Gokceada Island. Results showed that the fish fauna of the island was predominantly shaped by fish stocking activities.