In the title compound, [Mg(SO(4))(C(12)H(8)N(2))(H(2)O)(3)], the Mg(II) centre exhibits a slightly distorted octa-hedral coordination environment defined by two N atoms from a 1,10-phenanthroline mol-ecule, one O atom from a sulfate dianion and three meridionally arranged O atoms from coordinated water mol-ecules. The crystal structure involves intra- and intermolecular O-H⋯O hydrogen bonds.