A process for the recovery of potassium. magnesium and sulfate from seawater brine in the form of potassium sulfate magnesium sulfate double salt of fertilizer quality, substantially free of sodium chloride, sodium sulfate and other salts, comprising the steps of: (a) Concentrating seawater to a density between about 1.26 and about 1.315 specific gravity; (b) Adding to the said concentrated seawat