Grain crops have expanded at the expense of native grasslands in South America's Pampa biome in recent decades, thereby, increasing the sediment delivery to the river systems. This study aimed to evaluate the impact of land use change on sediment source contributions in the Ibirapuitã catchment (5942 km 2 ), Southern Brazil. For this purpose, a sediment fingerprinting approach was developed based on organic matter composition, ultraviolet–visible reflectance, and fallout radionuclide activities as potential tracers. Four main sediment sources were investigated: croplands ( n = 36), native grasslands ( n = 31), unpaved roads ( n = 31), and subsurface sources ( n = 34). Tracers were selected following a three‐step procedure: conservative range test, Kruskal–Wallis H‐test, and linear discriminant function analysis (LDA). Selected tracers were introduced into a mass balance mixing model to estimate the source contributions to in‐stream sediment. The seven tracers selected by the LDA were able to explain 91% of the variance and correctly classify 83% of the source samples. Despite covering less than 10% of the catchment surface area, croplands that replaced the native grasslands supplied the primary sediment source (33%) to the main outlet, followed by subsurface sources (27%). In contrast, native grasslands covering 80% of the surface area provided only 17% of sediment to the river network. These findings confirm that soil erosion processes are accelerating in response to the recent land use changes in the region. To prevent soil loss and sediment delivery to the river systems, land use conversion from native grasslands into croplands should be associated with the implementation of appropriate land conservation practices, such as runoff control, no‐tillage system, and crop rotation.