The metridioid enthemonaean sea-anemone family Gonactiniidae Carlgren, 1893 is characterized by having one of the simplest morphologies among the Actiniaria. It currently includes two monotypic genera, each consisting of a species described in the 19th century from the Atlantic, although additional specimens have also been reported from the Mediterranean and South Pacific. In this study, we discovered a new species of gonactiniid anemone from Kumano-Nada, off the Pacific coast of Japan. This is the first record of this family from the North Pacific. According to our phylogenetic analyses, this anemone is the sister group to the other two genera of Gonactiniidae, Gonactinia Sars, 1851 and Protanthea Carlgren, 1891. Accordingly, we establish a new genus for this new species. Additionally, we discuss the simplification of body form in this lineage of sea anemones based on the discovery of this new species, which exhibits an intermediate morphology between the other gonactiniids and non-gonactiniid actiniarians.