Reef proliferation and carbonate platform expansion took place during the Late Triassic. These major trends are mainly documented in the Tethys realm. The main goal of this study is to improve our understanding of mid-oceanic carbonate platforms of the West Panthalassa during the Late Triassic. Special focus is given to the shallow water depositional settings and to the ecological structure and evolution of reef ecosystems. We investigate Upper Triassic carbonates of the Sambosan Accretionary Complex in Southwest Japan. Detailed microfacies analysis has been carried out on thin sections from twenty four localities at Kyushu Island, Shikoku Island and Honshu Island. Our results highlight the evolution of mid-oceanic shallow water environments of the West Panthalassa domain during the Late Triassic and refine the correlation of the Sambosan limestone with coeval shallow water carbonates of the Tethys and East Panthalassa domain. This work provides valuable data to improve Early Mesozoic paleogeographic reconstructions.