The title compound, C15H10O2, crystallized in the centrosymmetric space group C2/c. Hydrogen bonding is of the cyclic dimer type about a center of symmetry. The carboxyl H and O atoms are ordered. The phenanthrene core is almost planar and the dihedral angle between the core plane and the carboxyl group plane is only 1.8(1) degrees. The bay H atoms, H4 and H5, lie close to the core plane and 2.02 (2) A from each other; the refinement places them on opposite sides of the core plane, but this result has little statistical significance.