A new butterfly taxon,Nacaduba sinhalaramaswamii ssp. nov.(Lepidoptera: Lycaenidae: Polyommatini),is described from Agasthyamalais of southern Western Ghats in peninsular India. The new taxa can be diagnosed from all other Nacaduba of southern India and N. sinhala Ormiston, 1924 from Sri Lanka, by its distinct male genitalia. The early stages, larval hostplants, flight periods, ecology and the known distribution of the new taxa are discussed. The revised keys to all known Nacaduba of Western Ghats of peninsular India are provided.