The Plio-Pleistocene Kazusa Group, situated in the middle and upper basin of the Tama River in Northwestern Tokyo, Japan is known to have yielded various vertebrates, including both terrestrial and marine mammals as well as chondrichthyan fishes. Recently, osteichthyan fish fossils have been discovered from this stratum. Notably, marine fish fossils have been discovered in the Oyabe, Hirayama, Komiya, Oyamada, and Renkoji formations of Kazusa Group. Of these formations, the Oyabe Formation has revealed Clupeiformes and the first East Asian fossils of the Plotosidae, while the Renkoji Formation has yielded Clupeiformes, Clupeidae, Engraulidae, and the second recorded occurrence of the Mugilidae in Japan. These recent discoveries suggest that the Kazusa Group in Northwestern Tokyo has the potential to be a significant stratum for the study of Quaternary marine fossil osteichthyan fishes.