During the late Mesozoic, Paleo-Pacific subduction exerted profound controls in shaping the tectonic evolution of the Southeast (SE) Asian continental margin. Nevertheless, the geodynamic mechanisms governing this subduction system, particularly its impact on the southeastern South China block margin, remain poorly constrained. Hainan Island, located in the southern South China block, preserves significant Late Jurassic−Early Cretaceous igneous rocks that offer valuable insights into the region’s tectonic history and the Paleo-Pacific subduction system. This study presents zircon U-Pb geochronologic and Hf isotopic data, whole-rock elemental data, and Sr-Nd isotopic analyses of the Late Jurassic diorites and Early Cretaceous mafic rocks from Hainan Island. The Late Jurassic diorites exhibit Nb-enriched signatures, positive ε_Hf(t) values (+3.16 to +10.04), and arc-like geochemical features, reflective of magma influenced by asthenosphere-lithosphere interaction during subduction retreat. The Early Cretaceous mafic rocks (ca. 121−100 Ma) belong to the calc-alkaline series and are subdivided into two groups: group A, marked by ocean-island basalt−like geochemical patterns and positive ε_Nd(t) values (+3.82 to +4.89), suggesting derivation from partial melting of asthenospheric mantle; and group B, showing arc-like geochemical signatures and lower ε_Nd(t) (−6.53 to −1.30) and ε_Hf(t) (−4.04 to +0.94) values, indicating an enriched mantle source with an addition of recycled sediment−derived fluids. Our results, when combined with previous data, reveal three major magmatic episodes in Hainan Island: Late Jurassic (162−158 Ma), Early Cretaceous (136−114 Ma), and Late Cretaceous (108−73 Ma). These magmatic phases coincide with coeval activity in the South China block coastal provinces and the Dalat zone in Vietnam. We propose that the late Mesozoic tectonic evolution of Hainan Island and the SE South China block reflects a phase of intracontinental orogenesis associated with a transition from low-angle to high-angle subduction, driven by changes in the Paleo-Pacific subduction regime.