The geotectonic boundary between the North China (Sino-Korean) and South China (Yangtze) cratons in mainland China forms a remarkable collisional suture, featuring the ca. 230 Ma (Middle Triassic) ultrahigh-pressure (UHP) metamorphic belt in the Dabieshan-Sulu area. Its eastern extension, however, becomes unclear, due to a large-scale tectonic offset related to the Miocene opening of the Bohai basin and Japan Sea. This article reviews the geological characteristics of the mid-Triassic medium-pressure (MP) metamorphic belts in the Korean peninsula and the Japanese Islands, in particular, their protolith assemblage, metamorphic age, and structural horizon. In the Korean peninsula, the Triassic MP metamorphic belt appears twice on the surface, i.e., along the Imjingan belt to the north and along the Ogchon zone to the south. In Japan, the mid-Triassic MP metamorphic belt occurs in north-central Kyushu, in the eastern margin of the Hida mountains in central Japan, and in the northern Kanto area. We conclude that both the MP metamorphic belts in the Korean peninsula and the Japanese Islands correspond to the eastern extension of the collisional suture between the North and South China cratons, on the basis of lines of evidence that include (1) coeval Triassic age of UHP and MP metamorphisms, (2) same protolith assemblage of shelf facies deposited along the passive continental margin, and (3) structural horizon between overlying North China rocks with an Archean heritage and underlying South China rocks with a Proterozoic affinity. This geotectonic correlation among mainland China, Korean peninsula, and the Japanese Islands, in particular, the documentation of the suture in Japan, up to the north of Tokyo, suggests that major parts of the Japanese Islands have developed along the South China margin, whereas the Hida and Oki belts form sole remnants derived from the North China craton.