A new species, Ranunculus kastamonuensis sp. nov. is described from Turkey. Diagnostic characters, description, illustration, taxonomic and a short biogeographic comment on the species are presented. Its relationships with R. peltata, R. rionii and R. trichophyllus are discussed. A map including the distribution of the species and other related species in Turkey is given. IUCN threatened category is proposed and discussed for the new species.