We describe a new species of ptychodactiarian sea anemone, Ptychodactis aleutiensis, and redefine the family Ptychodactiidae and the previously monotypic genus Ptychodactis to accommodate P. aleutiensis sp. nov. Individuals of the new species were photographed and collected at depths of less than 20 m off the Aleutian Islands, Alaska. Many were found detached, adrift in the water. The species differs from the only other member in its genus, Ptychodactis patula, in having more tentacles, tentacles only at the margin, frilled structures associated with only two siphonoglyphs, infertile primary mesenteries, oral stomata, holotrichous nematocysts of two size classes in the tentacles, actinopharynx, and mesenterial filaments, and by the morphology of the holotrichs and spirocysts. It is the fourth species of ptychodactiarian described; Ptychodactis is the only genus of suborder Ptychodacteae with more than one species.