<p>Carbonate weathering is crucial for regulating global C cycle and short-term climate change. Calcium (Ca) is one of the major elements involved in carbonate weathering, and studying complex carbonate weathering processes can be helped by Ca isotopes in river system. Here, elemental and Ca isotopic (<i>δ</i>⁴⁴/⁴⁰Ca) data for dissolved loads and suspended sediments from the Lancang River (Southeastern Tibetan Plateau) were determined. Dissolved <i>δ</i>⁴⁴/⁴⁰Ca values in river water (0.64%–1.05%) decreased downstream, and most values are close to 0.88% in the mainstream. Relatively, suspended sediments are enriched in light Ca isotopes (0.46%–0.75%), which is controlled by mineralogy (clay vs. carbonate) based on the Ca/Al mass ratios and Ca-Zn isotopes evidence. According to the forward model, carbonate and evaporite weathering contribute &gt;90% dissolved Ca to river water, while silicate weathering contributes only a minor fraction (2%–8%). Dissolved <i>δ</i>⁴⁴/⁴⁰Ca values are positively related to Sr/Ca, Mg/Ca ratios, and calcite saturation index, revealing the precipitation of secondary carbonate as a possible mechanism causing heavier Ca isotopes in river water. However, such an effect is not observed in the lowland tributaries, where river water with high <i>δ</i>⁴⁴/⁴⁰Ca and <i>δ</i>7Li values indicates the influence of secondary clay formation. After comparing Ca fluxes and dissolved <i>δ</i>⁴⁴/⁴⁰Ca values in large world rivers, we propose that riverine dissolved <i>δ</i>⁴⁴/⁴⁰Ca values are dominated by carbonate weathering and secondary carbonate precipitation. Riverine Ca isotopes could serve as a potential indicator to understand and model secondary processes during carbonate weathering in the surficial environment of Earth.</p>