The title compound, C20H17N3OS, is a 1H-pyrazol-3(2H)-one having aromatic 4-(1,3-benzothiazol-2-yl)- and 2-phenyl substituents. The five-membered ring and fused ring system are planar, the r.m.s. deviations being 0.021 and 0.005 , respectively. The five-membered ring is aligned at 7.9 (2)° with respect to the fused-ring system. The allyl and phenyl parts of the molecule are both disordered over two positions in a 1:1 ratio. Weak intermolecular C—H...O hydrogen bonding is present in the crystal structure.