Phanerozoic internal and peripheral orogens in Northeast Asia converge toward the Korean Peninsula situated between cratonic Asia and the outboard magmatic arc. Widespread Mesozoic plutons in the peninsula provide first-hand information about the magmatic response to the continental and oceanic plate subduction. The present study addresses this issue using comprehensive (n >1100) whole-rock geochemical, zircon U-Pb geochronological, and O-Hf isotopic data obtained from Triassic gabbro-pyroxenite-mangerite-monzonite-syenite-granodiorite-granite plutons in the central and southern parts of the peninsula. The intrusion of ca. 265−250 Ma calc-alkaline granitoids, including the high-silica adakite, along the outboard (in present coordinates) Yeongnam Massif is coeval with or slightly younger than the Barrovian metamorphism recognized in fold-and-thrust belts surrounding the inboard (northward, present coordinates) Gyeonggi Massif, suggesting a close link between the collisional orogenesis and subduction initiation as commonly documented in Phanerozoic supercontinents. Subsequent Late Triassic plutons emplaced in the Yeongnam Massif are subdivided into the older (232−224 Ma) magnesian and alkali-calcic to calc-alkalic group and the younger (220−217 Ma) ferroan and alkalic to alkali-calcic group temporally intervened by the geochemically arc-like Andong ultramafic complex (222 Ma). Zircon O-Hf isotopic compositions of the older plutons reflect the mixing of metasomatized lithospheric mantle, young (probably Paleozoic) arc crust, and Precambrian basement crust, whereas those of the younger plutons reflect input of the asthenospheric/lithospheric mantle and mafic lower crust. Meanwhile, the Late Triassic (233−224 Ma) potassic plutons in and around the Gyeonggi Massif represent post-collisional magmatism most likely induced by slab breakoff, which may also have been responsible for the shoshonitic magmatism in the Yeongnam Massif. Spatial differences in the age pattern and O-Hf isotopic signature of inherited and synmagmatic zircons from the potassic plutons indicate a selective contribution from an ancient metasomatized lithospheric mantle beneath the North China-like craton and an allochthonous South China-like lithosphere. The formation of the Triassic plutons could be explained by a series of tectonomagmatic events consisting sequentially of the ridge subduction, low-angle subduction, slab breakoff beneath the collisional orogen, tectonic switch to an extension-dominated arc system, and delamination of an overthickened arc lithosphere.