In the title isobenzo-furan-one derivative, C20H15NO2, the planar fused-ring system (r.m.s. deviation for the 10 fitted atoms = 0.031 Å) forms dihedral angles of 63.58 (6) and 63.17 (8)° with the N-bound phenyl rings; the dihedral angle between the planes of these phenyl rings is 85.92 (7)°. In the crystal, mol-ecules are linked by weak C-H⋯O inter-actions, involving both O atoms, forming helical supra-molecular chains along [001].