Sodium ammonium phosphate, also known as sodium ammonium biphosphate tetrahydrate, is an inorganic compound with the chemical formula H13NNaO8P. This compound is colorless crystals or white particles that are soluble in water and easily hygroscopic. Its pH value is alkaline and its solubility is good. At high temperatures, sodium ammonium biphosphate tetrahydrate loses its crystalline water and decomposes into ammonium biphosphate and sodium phosphite.

Sodium ammonium phosphate is commonly used as a chemical reagent in laboratories, and it can be used as a component of buffers, regulators, and precipitants in biochemistry, analytical chemistry, and pharmaceutical processes.