The Neoproterozoic oxygenation event (NOE) likely began earlier than 800 Ma, raising oxygen levels in the Earth’s atmosphere-ocean system and, thus, setting the stage for the emergence and diversification of animals. However, the redox history of the oceans during the following Cryogenian Period (∼720–635 Ma) remains poorly constrained. Here, we present a biogeochemical study based on Mo-isotope (this study) and Fe, C, Mo abundance data (previously reported data) for black shales of the ∼660-Ma Datangpo Formation in the Minle section of the Nanhua Basin, South China. Iron speciation data indicate that the study samples were most likely deposited under euxinic conditions. The studied samples yield two clusters of δ98Mo data: +0.98‰ to +1.14‰ [+1.06 ± 0.06 (SD) ‰] and −0.13‰ to +0.68‰ (+0.65 ± 0.31‰), respectively, with the higher values found mostly in the lowermost black shales. The variably lower δ98Mo values probably do not reflect contemporaneous open-ocean seawater composition owing to lower aqueous H2S concentrations caused by substantial watermass restriction (as evidenced by Mo-TOC covariation patterns), whereas the high δ98Mo values may record contemporaneous Cryogenian seawater values. The seawater δ98Mo values from our study at ∼660 Ma are similar to previously reported seawater δ98Mo values at ∼750 Ma (+1.11‰) and ∼640 Ma (+1.15‰), implying the existence of widespread oceanic anoxia throughout the Cryogenian. Because the subsequent Ediacaran Period shows evidence of greater oceanic oxygenation based on multiple geochemical proxies, our findings suggest that the process of oceanic oxygenation proceeded slowly during the Cryogenian Period. This may have been an important factor in delaying the first appearance of metazoans until the late Cryogenian and their diversification until the Ediacaran.