The title compound, (NH(4))(3)[GaMo(6)(OH)(6)O(18)]·7H(2)O, contains two centrosymmetric GaMo(6) B-type Anderson cluster units consisting of central GaO(6) octa-hedra surrounded by a hexa-gonal assembly of MoO(6) edge-sharing octa-hedra. Like other B-type Anderson clusters, where the central Mo atom is substituted with a di- or trivalent metal ion, the central six μ(3)-oxido bridges are protonated. The average Ga-O bond length is 1.97 (1) Å, whereas the average Mo-O distances are 2.29 (2), 1.94 (1) and 1.709 (5) Å, respectively, for Mo-(μ(3)-OH), Mo-(μ(2)-O) and Mo=O bonds. In the crystal structure, the Ga(μ(3)-OH)(6)Mo(6)O(18) (3-) polyanionic clusters are surrounded by NH(4) (+) cations and solvent water mol-ecules, forming an extended network of hydrogen bonds.