A new species, Persicaria geocarpica, from central Honshu, Japan, is described. The plants are known from only a few sites in northeastern Aichi Prefecture nearby the type locality where they were first discovered. After detailed and intensive field studies, it became clear that the habitat of P. geocarpica is restricted to a particular mountainous area in the Tokai district. Persicaria geocarpica exhibits the unique characteristic that the inflorescences are elongated down to ground level at the end of the long flowering season. There are almost no morphological differences between terrestrial and subterranean flowers, but the latter can self-pollinate in the bud and mature underground without the floral parts expanding.