A new species of Actinostemon is described and illustrated. Actinostemon roselii is mentioned only for the states of Paraná and Santa Catarina (Brazil), where it grows in riparian tropical rainforests. This species can be recognized by its conspicuous and depressed ovate bracts of the staminate cymule, foliar buds globoid, staminate cymule with three or four flowers, and staminate and pistillate flowers monochlamydeous. Comments about its distribution and ecology are provided, as well as a key to identify the Brazilian species of Actinostemon. Based on IUCN criteria, A. roselii can be considered a data deficient (DD) species.