Doridicola similis n. sp. (Poecilostomatoida, Rhynchomolgidae) is described from specimens found on the gills of a squid (Sepiotheuthis lessoniana Lesson) caught in the Gulf of Thailand. It is close to its congeners D. longicauda (Claus, 1860) and D. sepiae (Izawa, 1976) but can be distinguished from them by the possession of longer caudal rami. A cladistic analysis of the genus revealed that the 12 species of Doridicola associated with mollusks (Gastropoda, Bivalvia, and Cephalopoda) are on the same clade that diverged early in cladogenesis from those species occurring mostly on the octocorallian (alcyonacean) cnidarians. The three species occurring on cephalopods are monophyletic and closely allied with D. agilis Leydig, 1853, occurring on nudibranchs in Europe.