The synthesis and molecular structure of a monomeric diplatinum(II) complex composed of mono-lacunary α-Keggin polyoxometalate is described. The polyoxometalate, Cs₃[α-PW₁₁O₃₉{cis-Pt(NH₃)₂}₂]·8H₂O (Cs-1), afforded by a stoichiometric reaction of mono-lacunary Keggin polyoxotungstate with cis-diamminedichloroplatinum(II) in water, followed by crystallization from water, was obtained as analytically pure, homogeneous, yellow crystals. The compound Cs-1 was characterized by elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR) and UV-visible spectroscopy, solution ¹H and ³¹P nuclear magnetic resonance (NMR), and X-ray crystallography. The single-crystal X-ray structure analysis revealed that the two cis-platinum(II) moieties, [cis-Pt(NH₃)₂]²⁺, were coordinated each to two oxygen atoms in a mono-vacant site of [α-PW₁₁O₃₉]⁷⁻ with asymmetric configuration, resulting in an overall C₁ symmetry. Furthermore, hydrogen evolution from an EDTA·2Na (ethylenediamine tetraacetic acid disodium salt) aqueous solution under visible-light irradiation (≥400 nm) was achieved by using polyoxoanion 1 and titanium dioxide.