Silicon isotope values (d30SiDSi) of dissolved silicon (DSi) have been analyzed in the Lena River and its tributaries, one ofthe largest Arctic watersheds in the world. The geographical and temporal variations of d30SiDSi range from +0.39 to +1.86‰with DSi concentrations from 34 to 121 lM. No obvious patterns of DSi concentrations and d30SiDSi values were observedalong over 200 km of the two major tributaries, the Viliui and Aldan Rivers. In summer, the variations of DSi concentrationsand d30SiDSi values in the water are either caused by biological uptake by higher plants and phytoplankton or by mixing ofwater masses carrying different DSi concentrations and d30SiDSi values. DSi in tributaries from the Verkhoyansk MountainRange seems to be associated with secondary clay formation that increased the d30SiDSi values, while terrestrial biological productionis likely more prevalent in controlling d30SiDSi values in Central Siberian Plateau and Lena Amganski Inter-RiverArea. In winter, when soils were frozen, the d30SiDSi values in the river appeared to be controlled by weathering and clay formationin deep intrapermafrost groundwater. During the spring flood, dissolved silicate materials and phytoliths were flushedfrom the upper thawed soils into rivers, which reset d30SiDSi values to the values observed prior to the biological bloom insummer. The results indicate that the Si isotope values reflect the changing processes controlling Si outputs to the Lena Riverand to the Arctic Ocean between seasons. The annual average d30SiDSi value of the Lena Si flux is calculated to be +0.86± 0.3‰ using measured d30SiDSi values from each season. Combined with the estimate of +1.6 ± 0.25‰ for the Yenisey River,an updated d30SiDSi value of the major river Si inputs to the Arctic Ocean is estimated to be +1.3 ± 0.3‰. This value isexpected to shift towards higher values in the future because of the impacts from a variety of biological and geochemicalprocesses and sources under global warming.