In the organic molecule of the title hydrate, C11H15N3OS·H2O, {systematic name: 3-ethyl-1-{(E)-[1-(2-hydroxyphenyl)ethylidene]amino}thiourea monohydrate}, a dihedral angle of 5.39 (2)° is formed between the hydroxybenzene ring and the non-H atoms comprising the side chain (r.m.s. deviation = 0.0625 Å), with the major deviation from planarity noted for the terminal ethyl group [the C—N—C—C torsion angle = −172.17 (13)°]. The N—H H atoms are syn and an intramolecular hydroxy–imine O—H...N hydrogen bond is noted. In the crystal, the N-bonded H atoms form hydrogen bonds to symmetry-related water molecules, and the latter form donor interactions with the hydroxy O atom and with a hydroxybenzene ring, forming a O—H...π interaction. The hydrogen bonding leads to supramolecular tubes aligned along the b axis. The tubes are connected into layers via C—H...O interactions, and these stack along the c axis with no directional interactions between them.