Electrochemical oxidation of the tetrathiafulvalene (TTF) type organic donor bis(ethylenediseleno)tetrathiafulvalene (BEST) in the presence of the Keggin polyoxometalate [PMo12O40](3-) affords the radical salt formulated as (BEST)(3)H[PMo12O40] (crystal data : triclinic, space group P1 with a = 13.056(1) Angstrom, b = 13.957(1) Angstrom, c = 22.302(3) Angstrom, 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 [PMo12O40](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).