The Co(2+) ion in the title compound, Rb[Co(C3H7NO3)(H2O)5][H6CoMo6O24]·C3H7NO3·10H2O, is coordinated by five water mol-ecules and one O-monodentate l-serine ligand in a slightly distorted octahedral geometry. The Rb(+) ion is irregularly coordinated by nine O atoms. In the crystal, the [H6Co(III)Mo6O24](3-) polyanions are stacked along the b-axis direction, mediated by bridging Rb-O bonds. N-H⋯O and O-H⋯O hydrogen bonds are observed involving the l-serine mol-ecules.