Monomagnesium phosphate is one of the forms of magnesium phosphate. It is a magnesium acid salt of phosphoric acid with the chemical formula Mg(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub>.<br />Di- and tetrahydrates are known also. It dissolves in water, forming phosphoric acid and depositing a solid precipitate of Mg(HPO<sub>4</sub>)÷3H<sub>2</sub>O, dimagnesium phosphate.
As a food additive, it is used as an acidity regulator and has the E number E343.