3-Carb­oxy­methyl-1H-indole-4-carb­oxy­lic acid

In the title compound, C₁₁H₉NO₄, the carboxyl group bonded to the six-membered ring lies close to the plane of the 1H-indole ring system [dihedral angle = 13.13 (9)°], whereas the carb­oxy­lic acid group linked to the five-membered ring by a methyl­ene bridge is close to perpendicular [78.85 (9)°]. In the crystal, O—H...O and N—H...O hydrogen bonds link the mol­ecules, generating (110) sheets.