In the title hydrate, C9H12N4S·H2O (systematic name: 3-methyl-1-{(E)-[(3-methylpyridin-2-yl)methylidene]amino}thiourea monohydrate), a small twist is noted between the pyridine ring and the rest of the organic molecule [dihedral angle = 6.96 (5)°]. The imine and pyridine N atoms are syn, and the amine H atoms are anti. The latter arrangement allows for the formation of an intramolecular N—H...N(imine) hydrogen bond. Both the N-bonded H atoms form hydrogen bonds to symmetry-related water molecules, and the latter forms O—H hydrogen bonds with the pyridine N and thione S atoms. These interactions lead to supramolecular layers that stack along the a-axis direction with no specific interactions between them.