Cryogenian Sturtian and Marinoan glacial and associated strata record the most severe paleoclimatic events in Earth history and are the main test ground of the snowball Earth hypothesis (SEH). The SEH predicts that both Sturtian and Marinoan deglaciations were globally synchronous, rapid, and catastrophic. The sharp transition from glacial diamictite to the overlying cap dolostone represents a key event during these deglaciations. Thus, a positive test for globally synchronous deglaciation requires high-precision radiometric dates to tightly bracket this transition on each paleocontinent and within every substantial sedimentary basin; however, such high-resolution dates are scarce and have limited stratigraphic and paleogeographic coverage. Here, we report two high-precision U-Pb zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 658.80 ± 0.50 Ma and 634.57 ± 0.88 Ma from tuffaceous layers that occur, respectively, within the cap dolostone atop the Tiesi’ao diamictite (Sturtian age) and at the topmost Nantuo diamictite (Marinoan age) in South China. The 658.80 ± 0.50 Ma age represents a high-precision minimum age constraint on the termination of the Sturtian-age glaciation. The 634.57 ± 0.88 Ma age and a previously published age of 635.23 ± 0.57 Ma from the topmost cap dolostone are indistinguishable within uncertainty, and together they provide tight constraints on the termination of the Marinoan glaciation in South China at ca. 635 Ma and directly bracket the duration of the cap dolostone to be <106 yr. The new data support the rapid termination of the Marinoan glaciation in South China and are consistent with global synchroneity of Cryogenian deglaciation events.