Based on morphology and chromosome numbers, Arisaema inaense (Seriz.) K. Sasamura et J. Murata is newly recognized at the rank of species and a new species A. nagiense T. Kobayashi, K. Sasamura et J. Murata is described. Both are diploid with 2n=26 and lack the accessory buds that always accompany the axillary buds of tetraploid plants of A. ovale Nakai. In the A. ovale group, A. nagiense is unique in the inflorescence unfolding before the leaf blade.