The asymmetric unit of the title complex, {[AgEr(C₅H₃N₂O₂)₂(C₂O₄)]·H₂O}_n, contains one Er^III atom, one Ag^I atom, two pyrazine-2-carboxyl­ate (pyc) ligands, two half oxalate ligands (each lying on an inversion center) and one uncoordinated water mol­ecule. The Er^III atom is coordinated by two O atoms and two N atoms from two pyc ligands, one O atom from a third pyc ligand and four O atoms from two oxalate ligands in a distorted monocapped square-anti­prismatic geometry. The Ag^I atom is coordinated by two N atoms from two pyc ligands, one O atom from a third pyc ligand and one O atom from one oxalate ligand. The crystal structure exhibits a three-dimensional heterometallic polymeric network. O—HO hydrogen bonding between the uncoordinated water mol­ecule and carboxyl­ate O atoms is observed.