The mixed-valence iron phosphate NH4Fe2(PO4)(2), which was synthesized by the high-temperature, high-pressure hydrothermal method, crystallizes in the monoclinic space group C2/c with a = 20.0066(1) Angstrom, b = 14.8319(2) Angstrom, c = 9.9899(1) Angstrom, beta = 119.278(1)degrees, and Z = 16. Its compact structure consists of chains of edge-sharing (FeO6)-O-II octahedra and chains of corner-sharing (FeO6)-O-III octahedra and PO4 tetrahedra. These chains an connected to each other and delimit tunnels in the [221], [2(2) over bar1$], and [001] directions, in which NH4+ cations are located. Mossbauer spectroscopy confirms the presence of Fe-II and Fe-III. The title compound is the first example of a mixed-valence ammonium iron phosphate.