The freshwater fish diversity of the Anatolian Midwestern Black Sea basin is studied. Twenty-seven species belonging to 18 genera and 9 families were recorded. The most representative families are Leuciscidae with eight species (30 %), followed by Cyprinidae with six species (22 %). The sub-basins sampled are clustered in three well-distinguished groups (Coastal Streams, Filyos and Kızılırmak) based on the similarity estimation of the species composition. The highest species richness was recorded in Kızılırmak River drainage, likely due to its greater size and higher habitat diversity. An updated set of fish occurrence data has been documented, resulting in new ranges of distribution and new records of species, accompanied by short comments.