In the title co-crystal, C10H8N2·C8H7NO4S, the formate group is coplanar with the pyridyl ring of the acid [dihedral angle = 6.2 (7)°], while the carboxymethylsulfanyl group makes a C—S—C—C torsion angle of 70.2 (1)° with the pyridine ring. The dihedral angle between the pyridyl rings of the 4,4′-bipyridine molecule is 27.4 (1)°. The acid and the 4,4′-bipyridine molecules are involved in hydrogen bonding via carboxylic O and pyridyl N atoms. The structure is further consolidated by intermolecular C—H...O hydrogen bonds, generating a three-dimensional network.