Ammonium iron(III) sulfate, NH4Fe(SO4)2·12 H2O, or NH4[Fe(H2O)6](SO4)2·6 H2O, also known as ferric ammonium sulfate (FAS) or iron alum, is a double salt in the class of alums, which consists of compounds with the general formula AB(SO4)2 · 12 H2O.

[6] It is a weak oxidizing agent, capable of being reduced to Mohr's salt, ferrous ammonium sulfate.

Equations for these conversions ignore the degree of hydration of the material.

Areas of use for FAS include waste water treatment,[7] tanning,[7] production of dyestuffs,[7] and as an etching agent in the production of electronic components.

[8] It has been used in a wide area of applications, including adiabatic refrigeration equipment,[9] biochemical analysis,[10] and organic synthesis.