We examined dicyemid mesozoans from the renal sacs of both Octopus vulgaris and Octopus minor, obtained off the coast of Japan, and found two new species that belong to the genus Dicyema. Dicyema japonicum sp. nov. from O. vulgaris, is a medium sized dicyemid, rarely exceeding 1500 µm in length. The number of peripheral cells in the vermiform phases is usually 22. The disc-shaped calotte and parapolar cells form the cephalic enlargement. The axial cell is cylindrical but is rounded anteriorly, and it extends forward to the base of propolar cells. Infusoriform embryos consist of 37 cells. In each of the four urn cells, there are the cell's own nucleus and one germinal cell with its own nucleus. Dicyema clavatum sp. nov. is a relatively small sized dicyemid, infrequently reaching 1000µm in length, and it is the first mesozoan species described from O. minor. The number of peripheral cells in the vermiform phases is usually 22. The calotte is cap-shaped and smoothly rounded. The axial cell is enlarged and rounded in the calotte region, and it extends anteriorly to the base of the propolar cells. Uropolar cells occasionally become verruciform. Infusoriform embryos are composed of 39 cells. Each of the urn cells contains two nuclei of its own and one germinal cell with its own uncleus. Further details relevant to the description of infusoriform embryos of Dicyema misakiense Nouvel et Nakao are provided and a note to Dicyema acuticephalum Nouvel is given. The dicyemid fauna in the two species of octopuses is briefly discussed.