The fauna of Iranian Dytiscidae (Coleoptera) is reviewed listing all hitherto known distribution records. Comments on taxonomy, identification and nomenclature are provided. The type locality is also given for each species originally described from Iran. One species, Rhantus (Rhantus) frontalis (Marsham, 1802) is new for the fauna of Iran. According to the literature and the new record, 140 dytiscid taxa representing 32 genera are at present known from Iran. Four species, Deronectes sahlbergi Zimmermann, 1932, Hydroporus nigrita (Fabricius, 1792), Hygrotus (Hygrotus) versicolor (Schaller, 1783), and Nebrioporus insignis (Klug, 1834) are doubtful. Five taxa, Agabus (Acatodes) elongatus (Gyllenhal, 1826), Coptotomus interrogatus (Fabricius, 1801), Hydaticus (Guignotites) fabricii (MacLeay, 1825), Hydroporus bodemeyeri bodemeyeri Ganglbauer, 1900, and Pachydrus sp. are excluded from the fauna of Iran.