The asymmetric unit of the title complex, {[AgEr(C(5)H(3)N(2)O(2))(2)(C(2)O(4))]·H(2)O}(n), contains one Er(III) atom, one Ag(I) atom, two pyrazine-2-carboxyl-ate (pyc) ligands, two half oxalate ligands (each lying on an inversion center) and one uncoordinated water mol-ecule. 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-anti-prismatic 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-H⋯O hydrogen bonding between the uncoordinated water mol-ecule and carboxyl-ate O atoms is observed.