The title compound, C9H7NO4, forms centrosymmetric dimers through intermolecular O-H⋯O hydrogen bonds in the crystal structure. The nitro group deviates slightly from coplanarity with the benzene ring. The benzene ring and the carboxylic acid group are in an E configuration about the ethylenic double bond. © 2005 International Union of Crystallography Printed in Great Britain - all rights reserved.