Ammonium orthomolybdate is the inorganic compound with the chemical formula (NH4)2MoO4.

It is a white solid that is prepared by treating molybdenum trioxide with aqueous ammonia.

Upon heating these solutions, ammonia is lost, to give ammonium heptamolybdate ((NH4)6Mo7O24·4H2O).

Ammonium orthomolybdate is used as a corrosion inhibitor and is an intermediate in some schemes to win molybdenum from its ores.

Heating ammonium orthomolybdate solid or treatment with acid gives molybdenum trioxide.