Humid tropical and subtropical regions commonly host thick soils that supply sediment to adjacent streams. These soils are depleted in Ca and Na and enriched in Al and Si, yielding high chemical weathering indices, including chemical index of alteration (CIA) values >90, indicative of intense chemical weathering. Here, we investigated chemical weathering in granitoid-sourced fluvial sediments of Puerto Rico—a region with one of the highest documented rates of chemical weathering on Earth. Although thick soils occur throughout the study area, weathering signatures from fluvial muds (CIA values of 58–69) are significantly lower than expected. Indeed, these weathering indices are similar to values expected in climates 10–20 °C cooler. These remarkably low values likely reflect significant input from partially weathered bedrock mobilized during mass wasting. Furthermore, future climate warming may increase torrential precipitation and accompanying mass wasting, producing a counterintuitive decrease in weathering intensity recorded in fluvial sediments.