The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait, reveal the presence of Proterozoic subcontinental lithospheric mantle beneath the highly extended southeast margin of the South China block. Both TRD (Re depleted) model ages for individual sulfides and model ages estimated from the initial ¹⁸⁷Os/¹⁸⁸Os ratios of Re-Os mixing lines require that some volumes of the subcontinental lithospheric mantle formed prior to 2.3–1.9 Ga. Later events in the subcontinental lithospheric mantle may be recorded by TRD model ages of 1.5–1.2 Ga and ca. 0.9 Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those known in the crust of the mainland South China block. The sulfide Os isotope data show that Proterozoic lithosphere beneath the South China block has survived the extensive Mesozoic Yanshanian magmatism on the continental margin and has not been delaminated even during the severe lithospheric extension that led to the subsidence of the Taiwan Strait.