The 9-cobalt(II)-containing trimeric, cyclic polyanion [Co9(OH)3(H2O)6(PO4)2(B-α-GeW9O34)3]21– (1) was synthesized in an aqueous phosphate solution at pH 8 and isolated as a hydrated mixed sodium–cesium salt. Polyanion 1 was structurally and compositionally characterized in the solid state by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, as well as thermogravimetric and elemental analyses. The magnetic and electrochemical properties of 1 were also studied and compared with those of its phosphorus analogue, [Co9(OH)3(H2O)6(HPO4)2(B-α-PW9O34)3]16– (Co9-P). The electrochemical water oxidation activity of the cesium salt of 1 under heterogeneous conditions was also studied and shown to be superior to that of Co9-P. The experimental results were supported by computational studies.