We present the first large dataset of dissolved silicon isotopes signatures (δ30Si) in different tropical rivers, including the Amazon and the Congo, the two largest silicon suppliers to the world ocean. A two-years long monthly series was obtained in the Congo River, upstream of the Kinshasa/Brazzaville urban zone. Spatial and temporal variations in the Amazon River and its main tributaries were studied for one year. Both the Congo and Amazon rivers convey similar mean δ30Si signatures to the ocean (close to +0.8 ), in the range of the few previously published data for those rivers. The Congo River exhibits limited seasonal variations, with the exception of some large δ30Si variations that seem related to the presence of high biogenic silicon content. This could be due to assimilation by diatoms in Pool Malebo, a lake-like widening in the lower reaches of the Congo River upstream of Brazzaville. In contrast, other rivers such as the Mekong (Vietnam) and the Tana (Kenya) can exhibit much heavier δ30Si values of up to +2 . The lightest δ30Si (close to 0 ) are observed in the Congo tributaries characterized by high organic content (black rivers).When comparing data across these different tropical rivers, we observe a strong correlation between δ30Si and pH that could reflect the link between Si isotopic fractionation and different processes including rock weathering and clays formation and dissolution.