To provide a method for producing 4-carboxy-1,8-octanedioic acid having good hue in high yield from 1,3,6-hexanetricarbonitrile obtained by the distillation refining of adiponitrile in a process to produce adiponitrile by the electrolytic dimerization reaction of acrylonitrile.

4-Carboxy-1,8-octanedioic acid is produced from a distillation residue containing 1,3,6-hexanetricarbonitrile produced by the distillation refining of adiponitrile in a process for producing adiponitrile by the electrolytic dimerization reaction of acrylonitrile by reacting the distillation residue with a lower alcohol, water and sulfuric acid to convert the nitrile to an ester of 4-carboxy-1,8-octanedioic acid, refining the ester by distillation and hydrolyzing the product in the presence of an acid catalyst.