Knowledge of the spin-density distribution in the dithiadiazolyl radical ring (DTDA) constitutes a major step towards the understanding of the magnetic and electronic properties of the rich magnetism of DTDA derivatives. The p-O2N·C6F4·CNSSN radical was chosen as the most favourable CNSSN derivative to study the spin distribution in this kind of free radical by polarised neutron diffraction. Spin-density maps obtained for the p-O2N·C6F4·CNSSN radical show that almost all the spin density is localised on the sulphur and nitrogen atoms of the CNSSN ring. A small negative spin density on the carbon atom of the CNSSN ring and a negligible spin density over the rest of the radical are observed, in good agreement with ab initio calculations.