A new large Chaeomysticeti indet., WMNH-Ge-1140240005 from the Shikiya Formation of Kumano Group (early middle Miocene; about 16 to 15 Ma) of Wakayama, Japan is described here. It preserves a large rostrum (about 50 cm width at the base of the rostrum), which has gently tapered lateral margins of the rostrum, narrow mesorostral groove at the level of the narial fossa, and wide premaxillae and maxillae. There are no diagnostic features pertaining to the specimen at the family level, but it is comparable to “cetotheres” sensu lato such as Pelocetus calvertensis and Diorocetus hiatus by having wide premaxillae that occupy 1/3 width of the rostrum anterior to the narial fossa in dorsal view, which implies that WMNH-Ge-1140240005 is a possible member of “cetotheres” sensu lato. Its size is possibly between the two large species Pelocetus calvertensis and Diorocetus hiatus of the early middle Miocene, and larger than the reported middle Miocene mysticete specimens from Japan (“Diorocetus” chichibuensis, “Diorocetus” shobarensis, Parietobalaena sp. (SMNH-VeF-62)). The rostral width of WMNH-Ge-1140240005 suggests that this animal was medium-sized compare to extant species, but the largest in the class of baleen whales of its time.