The title hemihydrate, C12H9N5·0.5H2O, was isolated from the condensation reaction of quinoline-2-carbaldehyde with 4-amino-4H-1,2,4-triazole. The Schiff base molecule adopts an E configuration about the C=N bond and is approximately planar, with a dihedral angle between the quinoline ring system and the 1,2,4-triazole ring of 12.2 (1)°. In the crystal, one water molecule bridges two Schiff base molecules via O—H...N hydrogen bonds. The Schiff base molecules are interconnected by π–π stacking interactions [centroid-centroid distances of 3.7486 (7) and 3.9003 (7) Å] into columns along [1\overline{1}0].
