Electrochemical oxidation of the tetrathiafulvalene (TTF) type organic donor bis(ethylenediseleno)tetrathiafulvalene (BEST) in the presence of the Keggin polyoxometalate [PMo(12)O(40)](3)(-) affords the radical salt formulated as (BEST)(3)H[PMo(12)O(40)] (crystal data: triclinic, space group P&onemacr; with a = 13.056(1) Å, b = 13.957(1) Å, c = 22.302(3) Å, alpha = 97.019(9) degrees, beta = 94.17(1) degrees, gamma = 95.847(9) degrees, and Z = 2). This is the first salt of a selenium-containing donor with a polyoxometalate cluster. The structure of this organic/inorganic hybrid consists of layers of the organic donors that alternate with polyoxometalate layers in the c direction. The organic molecules, which are completely ionized, form two types of layers with unprecedented packing arrangements and many intra- and interlayer side-on short contacts which give rise to a marked 3D electronic character. The magnetic susceptibility measurements indicate that the BEST(+) radicals are strongly antiferromagnetically coupled and that the polyanion has been reduced by one electron to give the mixed-valence anion [PMo(12)O(40)](4)(-). The ESR spectra show the progressive localization of the additional electron which becomes fully trapped on one of the Mo sites of the Keggin unit at low temperatures (T < 7 K).