We describe a new genus and species of hydrothermal vent sea anemone from the eastern north Pacific Ocean. The combination of characters in Alvinactis reu, gen. et sp. nov. is unique among currently known genera of the actiniarian superfamily Mesomyaria; most notable among its external features is a belt of verrucae and cinclides in the distal column. We assess the placement of Alvinactis, gen. nov. and evaluate taxonomic features used to distinguish groups within Actinostolidae Carlgren, 1893 and Actinoscyphiidae Stephenson, 1920 with a cladistic analysis of morphological characters. Phylogenetic analysis reveals that Alvinactis, gen. nov. and several genera previously ascribed to Actinostolidae belong in Actinoscyphiidae. Morphological evidence fails to support monophyly of Actinostolidae, but does support monophyly of the previously proposed subfamily Actinostolinae.