Huge amounts of mineral dust are produced in northern Africa, representing the largest source of aerosols worldwide. Transatlantic dust transport is known to fertilize soils as far as in the Amazon Basin. Yet, the influence of Saharan dust on chemical weathering fluxes and associated nutrient release in West Africa remains largely overlooked. To address this issue, we analysed clay fractions (<2 μm) of river sediments (n = 37) from across the Niger River basin - the largest river system in West Africa - using neodymium and hafnium isotope compositions as proxies for provenance (εNd) and chemical weathering (ΔεHf CLAY). Compared to previously published data for corresponding sand fractions, measured εNd values indicate significant size-dependent decoupling for Nd isotopes in most samples, with εNd differences between clay and sand fractions yielding values as great as ~26 ε-units. Using mixing models, we show that this discrepancy reflects the overwhelming presence in the studied clay fractions of Harmattan dust blown from the Bod´el´e Depression in Chad, which we estimate to account for about 40 % of the fine-grained sediment load exported to the Gulf of Guinea. Additionally, significant ΔεHf CLAY variability occurs across the Niger catchment, partly explained by the presence of zircon in clay-size fractions, but also by preferential alteration of dust-borne accessory phosphate minerals in the subtropical regions of the watershed. Based on these results, we propose that Saharan dust plays a major role in controlling regional patterns of chemical weathering in West Africa, suggesting that enhanced wet deposition of mineral dust in shield areas dominated by transport-limited weathering regime can result in a large increase in weatherability and associated release of phosphorus. These findings have general implications for the importance of mineral aerosols in controlling sediment yield and the supply of weathering-derived nutrients to continental areas bordering large subtropical deserts worldwide.