磷酸铵是磷酸根与铵根离子生成的化合物。按化合物中的氢、铵摩尔比，可分为以下三类：

• 正盐，氨与磷酸完全中和的产物，化学式(NH₄)₃PO₄。
• 磷酸氢二铵，又名磷酸氢铵、磷酸二铵，化学式(NH₄)₂HPO₄，由氨与磷酸2：1反应制得。
• 磷酸二氢铵，又名磷酸一铵，化学式NH₄H₂PO₄，由氨与磷酸1：2反应生成。

由于磷酸铵同时提供磷及氮，因此三种磷酸铵皆被用作肥料。

Ammonium phosphate is the salt of ammonium and phosphate. It is a highly unstable compound with the formula (NH₄)₃PO₄. Because of its instability, it is elusive and of no commercial value (except for scientific research). In addition to (NH₄)₃PO₄, a related double salt (NH₄)₃PO₄.(NH₄)₂HPO₄ is also recognized. It too is unstable. The instability of these salts results of their facile decomposition with evolution of ammonia: