The title compound, C(15)H(10)O(4)·H(2)O, also known as alizarin 1-methyl ether monohydrate, was isolated from Morinda officinalis How. The anthraquinone ring system is almost planar, the dihedral angle between the two outer benzene rings being 3.07 (4)°. In the crystal structure, O-H⋯O hydrogen bonds link the organic mol-ecules and the water mol-ecules, forming a three-dimensional network.