A new species, Pythium barbulae, was isolated from a non-symptomatic moss, Barbula unguiculata, from a road margin in Osaka, Japan. This description includes morphology, phylogeny, and pathogenicity to the host and vegetables. Pythium barbulae resembles P. cederbergense, P. iwayamai and P. violae, but differs in its smaller plerotic or nearly plerotic oospores. Inoculation experiments indicated that it was capable of latent infection in B. unguiculata stem-leaves and komatsuna seedlings without producing symptoms at 15 °C or 25 °C. Cucumber seedlings also had no symptoms at 25 °C but were caused significant damage at 15 °C by P. barbulae.