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The K isotopic composition of modern seawater is an important reference point to consider when using K isotopes to trace silicate weathering records in deep time; however, it is not yet well-constrained. The K isotopic composition of seawater also plays a pivotal role in understanding the global K budget, where seawater constitutes the isotopically heaviest terrestrial K reservoir at ~0.6‰ higher than common igneous rocks [[1]]. The high SP 41 sp K value of seawater obtained by our study suggests that K isotope fractionation during these processes needs further investigation to better constrain oceanic K-cycling. 3 (Color online) Potassium concentration [K] (ppm) plotted against the 41K (‰) of the mantle and the three primary K reservoirs: rivers, oceans and the upper continental crust. [Extracted from the article]