Many members of the bivalve superfamily Galeommatoidea have symbiotic associations with other marine benthic invertebrates. Among them, Koreamya arcuata (A. Adams, 1856) is distinctive because it is the only known bivalve symbiotic with brachiopods. Here we describe Koreamya setouchiensis n. sp. as the second example in this genus, based on specimens collected in and around the Seto Inland Sea, Japan. Similar to K. arcuata, this bivalve species attaches to the anterior end of the shell valve of living Lingula anatina Lamarck, 1801 by means of byssal threads. However, shell morphologies of the two bivalve species are clearly different; K. setouchiensis has an ovate shell, while K. arcuata has an elongated-triangular shell. These morphological differences are probably due to the difference in posture on the hosts. To understand how symbiotic association with Lingula evolved in Galeommatoidea, we performed molecular phylogenetic analyses using three nuclear (18S, 28S and H3) and one mitochondrial (COI) genes. The two Koreamya species with remarkably differently shaped shells were monophyletic, suggesting that their symbiotic associations with Lingula have the same evolutionary origin. Furthermore, the Koreamya clade formed a monophyletic group with anemone-associated galeommatoideans (Nipponomontacuta actinariophila and Montacutona sp.). This result and their morphological similarities suggest the possibility of host switching between sea anemones and Lingula.