The new radical ligand 5,8-dimethyl-1,4dioxonaphtho[2,3-d][1,2,3]dithiazolyl (1) is reported. Two crystal polymorphs, 1α and 1β, differing in their pancake-bonded dimerization motif and S•••O contact network, are identified. The self-assembly of Mn(II) metal ions with 1 leads to the formation of [Mn(hfac) 2 ] 3 (1) 2 that exhibits a Mn(II)−radical−Mn(II)−radical−Mn(II) linear arrangement of three Mn(hfac) 2 units bridged by two radical ligands (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-). Characterization by single-crystal X-ray diffraction of this Mn(II) complex packing structure reveals close noncovalent S•••O contacts between the [Mn(hfac) 2 ] 3 (1) 2 units in one dimension along the b−c direction. The magnetic properties of the coordination complex are characterized by dc and ac susceptibility measurements on a microcrystalline solid. The magnetic data down to 4.8 K indicate the presence of effective ferromagnetic interactions (J/k B = +0.16 K) between the molecular S T = 13/2 units along the supramolecular chain involving noncovalent S•••O contacts. Below 2.9 K, a non-zero out-of-phase component appears in the ac susceptibility, indicating the presence of a threedimensional magnetic phase transition.