In the title compound, [Ni(C15H11N3)2](ClO4)2·0.5H2O, the Ni2+ cation is coordinated by two terpyridine ligands to form a discrete complex and the coordination polyhedron can be described as a slightly distorted octahedron. It crystallizes as a hemihydrate with two perchlorate anions to compensate the charges. In the crystal, one of the two crystallographically independent perchlorate anions is involved in O—H...O hydrogen bonding to the water molecules, where two inversion-related water molecules link two inversion-related perchlorate anions into a ring with an R42(12) loop. The O-atom position of the water molecule is only half occupied, i.e. only half of the anions are involved in hydrogen bonding. A similar arrangement of two anions is also observed for the second crystallographically independent perchlorate anion but no water molecules are located between the anions. The cationic complex and the perchlorate anions are additionally linked by a number of weak C—H...O hydrogen bonds, forming a three-dimensional supramolecular structure. The crystal structure of the monohydrate of the same complex has been reported [Baker et al. (1995). Aust. J. Chem. 48, 1373–1378].