Abstract: [Objective] Sedimentological evidence for the coupled control of sea-level change and the East Asian monsoon on sediment provenance shift in small coastal bays remains limited. [Methods] This study takes surface sediments from bay-inflowing rivers and core sediments from borehole NDQK5 in Sansha Bay, Fujian Province as research materials. Combined with previously published AMS¹⁴C dating and micropaleontological identification results, the chronostratigraphic framework and provenance evolution sequence of the study area since the late MIS3 are established through clay mineral assemblage analysis. [Results] The results show that from the late MIS3 to MIS2 (core depth: 27.6–22.7 m), a closed freshwater lake developed in response to global sea-level fall, accompanied by a decrease in smectite content from 11.5% to 0%. Combined with the absence of smectite in modern surface sediments from non-tidal river reaches, this feature indicates that the exogenous sediment transport pathway was cut off, and sediment provenance was completely dominated by local fluvial input. In the early Holocene (Unit 2, before ~7.7 ka BP, subtidal to inner shelf environment), the average smectite content reaches 10.2%, and the clay mineral assemblage is consistent with the Yangtze River end-member, which may indicate that sediment supply was dominated by Yangtze-derived materials transported by coastal currents. In the middle Holocene (Unit 3, 7.7–7.0 ka BP, stable sea-level highstand period), samples show smectite depletion and a significant increase in chlorite content, corresponding to the maximum precipitation phase of the East Asian summer monsoon (EASM) when local watershed denudation intensity was enhanced. In the late Holocene (Unit 4, since ~7.0 ka BP, intertidal environment), with the weakening of the EASM, smectite content rebounds to 10.9%, and sediment provenance shifts back to Yangtze River dominance again. [Conclusion] Smectite content can serve as a proxy for distinguishing between distal Yangtze inputs and local watershed deposits. The sediment sources in Sansha Bay are jointly controlled by sea-level change and East Asian monsoon intensity, exhibiting a phased alternation pattern. [Significance] The sensitive sedimentary response of semi-enclosed bays to sea-level–monsoon coupling provides a scientific basis for predicting coastal sedimentation patterns under global warming and the sharp decline in Yangtze River sediment supply.