The asymmetric unit of the title compound, C10H24N3 +·0.5C4H2O4 2−·0.5C4H4O4, comprises a triisopropylguanidinium cation, half of a fumarate dianion and half of a fumaric acid molecule; both the fumarate dianion and the fumaric acid molecule are located on inversion centres. In the crystal, intermolecular O—H⋯O hydrogen bonds between the carboxyl groups of the fumaric acid molecules and the carboxylate groups of the fumarate anions lead to the formation of a hydrogen-bonded supramolecular twisted chain along the b axis. The triisopropylguanidinium cations interact with the fumarate–fumaric acid chains via extensive N—H⋯O and C—H⋯O hydrogen bonds, leading to a ladder arrangement, with the cation being the rungs that bridge three curled chains of fumarate–fumaric acid. The crystal packing is stabilized by N—H⋯O and C—H⋯O (cation⋯fumarate/fumaric) and O—H⋯O (fumarate⋯fumaric) hydrogen bonds, consolidating a three-dimensional network.