The asymmetric unit of the title compound comprises the monohydrated form of the natural product arcyriaflavin A [systematic name: 12,13-dihydro-5H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-5,7(6H)-dione monohydrate], C(20)H(11)N(3)O(2)·H(2)O. Individual molecular units are engaged in hydrogen-bonding interactions, forming two-dimensional zigzag supramolecular layers parallel to the (102) plane. The close packing of the layers is mediated by strong co-operative π-π stacking interactions, in tandem with interlayer hydrogen bonds involving the solvent water molecule.