In the title compound, (C(7)H(11)N(2))[Er(H(2)O)(8)]Cl(4)·H(2)O, the asymmetric unit consists of one 4-(dimethyl-amino)-pyridinium and one octa-aqua-erbium cation balanced by four Cl(-) anions, and one water mol-ecule. The 4-(dimethyl-amino)-pyridinium cation is protonated at the pyridine N atom. The dimethyl-amino group (C/N/C) lies close to the plane of the pyridinium ring, making a dihedral angle of 4.5 (3)°. In the crystal, the [Er(H(2)O)(8)](3+) cations are linked via O-H⋯O and O-H⋯Cl hydrogen bonds, forming two-dimensional networks propagating in the ab plane. These networks are linked via O-H⋯O and O-H⋯Cl hydrogen bonds, forming a three-dimensional network. The 4-(dimethyl-amino)-pyridinium cations are located in the cavities and are linked to the framework via N-H⋯Cl, C-H⋯O and C-H⋯Cl hydrogen bonds.