In the title compound, C(17)H(12)O(4)·H(2)O, the coumarin ring system is approximately planar with a maximum atomic deviation of 0.011 (2) Å, and is nearly perpendicular to the phenyl ring at a dihedral angle of 86.63 (9)°. In the crystal, mol-ecules are linked by classical O-H⋯O and weak C-H⋯O hydrogen bonds. π-π stacking is also present [centroid-centroid distance = 3.6898 (12) Å].