Riverine sulfate, primarily derived from the weathering of continental sulfur-bearing minerals, plays a key role in shaping the isotopic composition of marine sulfate. In arid regions, gypsum dissolution is typically assumed to be the dominant sulfate source, but conventional sulfur and oxygen isotope systems (delta 34S-SO4 2- and delta 18O-SO4 2-) struggle to distinguish it from overlapping atmospheric and anthropogenic sources. Here, we apply the triple oxygen value (Delta ' 17O-SO4 2-) in the Yellow River Basin-characterized by widespread agriculture, abundant gypsum, and minimal atmospheric deposition-to resolve these ambiguities. We observe strongly negative Delta ' 17O-SO4 2- values (as low as -0.3 parts per thousand) in upper basin waters during high-discharge periods, which we attribute to fertilizer-derived sulfate flushing from agricultural soils. These negative anomalies diminish downstream, where gypsum contributions become dominant. Isotopic modeling reveals that traditional approaches may overestimate gypsum inputs by up to 40%. Our results demonstrate that Delta ' 17O-SO4 2- is a powerful tracer of anthropogenic sulfate inputs in modern rivers. Moreover, its nonconservative behavior during transport challenges the assumption that negative Delta ' 17O values in paleo-sulfate archives strictly reflect contributions of atmospheric oxygen to sulfate.