Abstract: Chemical weathering of silicate minerals is an important source for Ca, Mg, Na and K. However, in comparison with other major elements (e.g., Ca, Mg and Na) in waters, how K behaves during water-rock interaction remains poorly understood. Recent studies have shown that large K isotopic fractionation could occur during various processes of low-temperature water-rock interaction such as chemical weathering and adsorption, making K isotopes gradually become a powerful tracer for the sources, migration and transformation of K cycling in the subsurface. This overview summarizes K isotopic compositions of major reservoirs at the Earth’s surface, including upper continental crust, hydrosphere and other reservoirs (plants and fertilizers). We conclude that ⁴¹K is enriched in hydrosphere than upper continental crust, providing an opportunity to identify the K source in groundwater.The magnitudes and mechanisms of K isotope fractionation during common water-rock interaction processes are also summarized (i.e., silicate dissolution, secondary mineral formation, adsorption, cation exchange), demonstrating that limited K isotope fractionation occurs during silicate dissolution, while clay formation results in enrichment of ⁴¹K in waters and adsorption and cation exchange leads to depletion of ⁴¹K occurring in waters. These different behaviors of K during these water-rock interactions provide an opportunity for tracing the migration and transformation process of K in groundwater. This paper presents the latest research that applied K isotopes to trace silicate weathering and water pollution. Since K isotopes are an excellent tracer for silicate weathering, they can be used to reveal the sources, migration and transformation of K cycling in aquifers with abundant CO₂. Additionally, the distinguishable behavior of K isotopes during chemical weathering, clay adsorption and cation exchange can be used to identify various water-rock interactions. Future K isotopic studies in the field of hydrogeochemistry should focus on: (1) constraining the contribution of multi-endmember control on sources of potassium in groundwater; (2) quantifying K behavior during long-term groundwater circulation; and (3) using multiple isotopes to trace carbon cycle-related processes.