Provenance changes and tectonic activity during the Early Oligocene in the Qiongdongnan Basin are responses to the evolution of the northwestern South China Sea. However, the provenance of the Lower Oligocene (Yacheng Formation) in the Qiongdongnan Basin is poorly understood owing to a lack of available borehole samples and poorly constrained stratigraphy. Here, combined with published data from wells of different part of the basin, we apply U–Pb dating of detrital zircons on samples nearby the western boundary fault of the basin to analyse the provenance of the Yacheng Formation and its regional tectonic implications. The zircon U–Pb age spectra of the sediments in three different members of Yacheng Formation show different features. A single U–Pb age peak at ca. 235 Ma occurs in the Third Member of the Yacheng Formation, whereas the U–Pb age spectra in the stratigraphically younger Second Yacheng Formation show different zircon U–Pb age spectra features: an age pattern has two major peaks at ca. 248 Ma and 441 Ma and lacks zircons younger than 200 Ma, and another pattern has two major peaks at ca. 95 Ma and 252 Ma. The Early Oligocene provenance evolution of the western Qiongdongnan Basin is reconstructed by comparing the zircon U–Pb age spectra of the sediments in the different samples to potential sources nearby. Abrupt changes in the U–Pb ages throughout the Lower Oligocene in the western Qiongdongnan Basin suggest that sediment supply in the Second Member of Yacheng Formation was dominated by rivers from western Hainan Island and Central Vietnam. By contrast, Hainan Island and local structural highs within the Qiongdongnan Basin were the main sources in the Third and First Members of the Yacheng Formation, respectively. The difference in the U–Pb age spectra between sediments in the Qiongdongnan Basin and the Red River implies that the time at which the Red River system entered the Qiongdongnan Basin was later than the Early Oligocene. Paleorivers sourced from Central Vietnam entered into the western Qiongdongnan Basin and did not reach the central or eastern part of the basin until ca. 31.5–29.9 Ma. An examination of the activity of the Red River Fault showed that the transition of provenance in the study area during the Early Oligocene could be a response to sinistral strike-slip motion along the Red River Fault. • U–Pb age spectra represent a prominent difference in the Lower Oligocene. • Sediments were sourced from local structural highs, Hainan Island and Central Vietnam. • Provenance changes probably correspond to sinistral strike-slip of the Red River Fault. • The Red River did not enter the Qiongdongnan Basin in the Early Oligocene.