The title compound, C5H6N4O, is approximately planar, with an angle of 11.04 (15)° between the planes of the pyrimidine ring and the non-H atoms of the carboximidamide unit. The mol-ecule adopts an E configuration about the C=N double bond. In the crystal, adjacent mol-ecules are linked by pairs of N-H⋯O hydrogen bonds, forming inversion dimers with an R 2 (2)(10) ring motif. The dimers are further linked via N-H⋯N and O-H⋯N hydrogen bonds into a sheet structure parallel to the ac plane. The crystal structure also features N-H⋯O and weak C-H⋯O hydrogen bonds and offset π-π stacking inter-actions between adjacent pyrimidine rings [centroid-centroid distance = 3.622 (1) Å].