Climate variability and anthropogenic environmental change are major drivers of sedimentary processes in the marginal seas of the western Pacific. However, distinguishing their respective influences remains challenging. To address this issue, clay mineral compositions, Ti/Ca ratios, and mean grain-size records spanning the last ~250 years were analyzed from sediment cores D0 and Y22 recovered from the northern South China Sea shelf and the Taiwan Strait, respectively. Clay mineral assemblages indicate distinct sediment sources for the two study areas. Sediments in Core D0 are primarily derived from the Pearl River and adjacent rivers, whereas Core Y22 records contributions from the Yangtze River, rivers of Taiwan, and the Zhejiang–Fujian coastal river systems. The sediment records reveal contrasting temporal patterns before and after the mid-twentieth century. In Core D0, increasing Ti/Ca and kaolinite/(illite + chlorite) ratios prior to the 1920s suggest enhanced terrigenous sediment delivery associated with intensified East Asian summer monsoon conditions. The relationship between climate and sedimentary proxies changed after the 1950s. Both study areas exhibit declining Ti/Ca ratios, indicating reduced terrigenous sediment input, accompanied by progressively coarser grain sizes. This pattern is inconsistent with the expected response to monsoon variability and suggests increasing anthropogenic modification of sediment supply, particularly through large-scale reservoir construction and hydraulic engineering projects. Similar trends observed in the Taiwan Strait imply that this pattern reflects regional-scale alterations in sediment supply rather than local depositional processes.