Lithium (Li) isotopes are a promising tracer of chemical weathering processes for both modern and ancient times. In order to improve the use of Li isotopes in the sedimentary record, here we calibrate the relationship between weathering intensity and detrital Li isotope composition (δ7Li) using the fine fraction of modern large river sediments. Through independent estimates for sediment provenance to calculate the Li isotope signature of the rock from which the sediments derive through weathering, we show that source rock variability (in particular the relative contribution of sedimentary versus igneous rocks) must be corrected for before using Li isotopes as a weathering proxy. We also show that for rivers draining mountain ranges, the contribution to river sediments of particles derived from sedimentary rocks is correlated to their Li/Al ratio, making it possible to use Li contents to estimate the average source rock composition. Once corrected for bedrock variability, the Li isotope signature defines a negative relationship with the weathering intensity (ratio between silicate weathering rate and total denudation rate), with highest Li isotope fractionation for the highest weathering intensity. Altogether, we propose a set of new relationships between weathering, erosion, provenance, and Li isotopes that can be used to quantify present-day and paleo-weathering using detrital sediment.