The title hydrated salt, C8H11N2O(+)·C2HO4(-)·H2O, was synthesized by a reaction of 4-meth-oxy-benzamidine (4-amidino-anisole) and oxalic acid in water solution. In the cation, the amidinium group forms a dihedral angle of 15.60 (6)° with the mean plane of the benzene ring. In the crystal, each amidinium unit is bound to three acetate anions and one water mol-ecule by six distinct N-H⋯O hydrogen bonds. The ion pairs of the asymmetric unit are joined by two N-H⋯O hydrogen bonds into ionic dimers in which the carbonyl O atom of the semi-oxalate anion acts as a bifurcated acceptor, thus generating an R(1)2(6) motif. These subunits are then joined through the remaining N-H⋯O hydrogen bonds to adjacent semi-oxalate anions into linear tetra-meric chains running approximately along the b axis. The structure is stabilized by N-H⋯O and O-H⋯O inter-molecular hydrogen bonds. The water mol-ecule plays an important role in the cohesion and the stability of the crystal structure being involved in three hydrogen bonds connecting two semi-oxalate anions as donor and a benzamidinium cation as acceptor.