A new species of dicyemid mesozoan is described from Sepiella japonica Sasaki, 1929, collected from the eastern area of Inland Sea, Osaka Bay, and Kii Strait in Japan. Dicyema sepiellae n. sp. is a large species that reaches about 4,000 µm in length. This species lives in folds of the renal appendages. The vermiform stages are characterized as having 24–29 peripheral cells, a conical calotte, and an axial cell that extends to the middle of the metapolar cells, but to the base of propolar cells in vermiform embryos. Infusoriform embryos consist of 39 cells; 2 nuclei are present in each urn cell and the refringent bodies are solid. This is the first description of a dicyemid in any species of Sepiella.