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While mountainous rivers in subtropical East Asia deliver a disproportionately large amount of sediments to the global ocean, the controlling mechanisms for sediment supply and transport remain understudied. Here we target a mesoscale tectonically stable mountainous river, the Minjiang River, in southeast China. We present petrography, heavy minerals, and Sr‐Nd isotopic data from suspended particulate matter (SPM) and riverbed sediments to characterize sediment‐climate feedback processes. Results show Sr‐Nd isotopic compositions of the SPM vary seasonally, corresponding well with the spatiotemporal variations of precipitation. River sands display low compositional and textural maturity and represent first cycle‐dominated and fast‐transported detritus. Provenance analysis suggests prominent contributions of upstream Proterozoic‐Paleozoic rocks to downstream SPM and riverbed sediments. We propose that climate‐induced hydrological variations exert a major control on sediment supply, transport, and compositions. Our study highlights the crucial role of climate in driving physical erosion of mountains in tectonically stable, subtropical East Asia. Plain Language Summary: The subtropical‐tropical rivers in East and South Asia, particularly those draining mountains, account for a significant proportion of global sediment and nutrient exports. Based on tectonic settings, river basin morphology, climatic conditions, and extents of anthropogenic impacts, two distinct types of sediment supply and transport regimes in the low‐middle latitudes of East Asia have been identified. Type 1 rivers are large rivers on the tectonically stable continent with slow, multicycled sediment transport, modulated by widespread flood plains, lakes, and human activities. Type 2 rivers are small mountainous rivers on tectonically active plate boundaries with rapid sediment transport, which are commonly triggered by extreme tectonic and climatic events. However, sediment transport mechanisms of the mountainous rivers in the tectonically stable, subtropical regions remain highly enigmatic. We combined petrology and geochemistry methods to track sediment sources of the Minjiang River (southeast China) and to characterize river sediment transfer processes. We find that hydroclimate changes, rather than anthropogenic forcings, exhibit the major control on discharged sediment compositions. We suggest that the mesoscale, tectonically stable, mountainous rivers have similar rapid sediment transport behavior to those small mountainous river systems, unlike other large river systems in subtropical East Asia. Key Points: Petrography, heavy mineral, and Sr‐Nd isotopic data were used to trace sediment provenance for a tectonically stable mountainous riverProvenance data indicates rapid sediment delivery in response to hydrological changes over seasonal timescalesClimate variability exerts a major control on sediment compositions, revealing a third type of sediment transport regime in East Asia [ABSTRACT FROM AUTHOR]