Seawater can be used as an alternative to freshwater for preparing cement-based materials. The effect of seawater on properties and hydrates of the magnesium potassium phosphate (MKP) cement pastes with Mg/PO molar ratio of 8 and low w/c ratio of 0.15 were investigated in this study. Compared with freshwater, seawater prolongs the MKP cement setting, slows down the heat release, increases the compressive strength in air, but slightly decreases the water resistance. K-struvite is the main hydrate in both pastes together with a minor amount of bobierrite and traces of MgKH(PO)⋅15HO in seawater. Seawater leads to less K-struvite, which agrees well with thermodynamic modelling predictions, but it also leads to partial elongated K-struvite needle crystals, which could contribute to the increased compressive strength. Water exposure has little changes on the main hydrates, but caused minor additional potassium chloride in the seawater-mixed paste, which partially explains the higher strength loss.