A layered ammonium nickel molybdate was prepared by precipitation from a solution of nickel nitrate and ammonium heptamolybdate. The compound obtained, (NH4)HNi2(OH)2(MoO4)(2), is trigonal with hexagonal unit cell parameters a = 6.0147(4) Angstrom, c = 21.8812(13) Angstrom, and Z = 3. A powder X-ray diffraction pattern was obtained using synchrotron radiation. The structure was generated from three-dimensional Patterson and difference Fourier density maps and refined in the space group R(3) over barm$ by the Rietveld method. The structure consists of molybdate tetrahedra and nickel octahedra forming layers perpendicular to the c axis. There are three layers per unit cell, with ammonium ions incorporated between the layers. The structure is a member of a solid solution series of (NH4)H2xNi3-xO(OH)(MoO4)(2), where 0 less than or equal to x less than or equal to 3/2. The nonstoichiometry arises from variable occupancy of the nickel position and allows for the formation of these compounds having Ni/Mo ratios varying from 0.75 to 1.5.