<p>Isactinernus quadrilobatus Carlgren, 1918, the only species of genus Isactinernus, is characterized by an oral disc with four equally-sized lobes. In 2003 Synactinernus flavus Carlgren, 1918, which is characterized by four large and four small lobes alternately arranged, was synonymized into I. quadrilobatus because the difference of lobe shape was thought to be intraspecific variation, and thus the genus Synactinernus was also synonymized into Isactinernus. However, our molecular phylogenetic analysis of Actinernidae including I. quadrilobatus collected from the Kumano Sea revealed that I. quadrilobatus is genetically distinct from S. flavus. Our morphological comparison between the two species supported the result of the molecular analysis. Consequently, the genus Synactinernus was re-established and separated from Isactinernus. Additionally, during this study, we discovered and described an additional species of the genus, Synactinernus churaumi Izumi and Fujii, 2019.</p>