A genuine organic coexisting system of magnetism and conductivity has been realized. For this purpose, organic donor molecules carrying stable radicals in a cross-π-conjugated manner, so called “Spin-Polarized Donors”, have been developed. Among them, ESBN, consisting of a benzo-annulated selenium-containing TTF skeleton and a nitronylnitroxide moiety, was synthesized and its ionradical salt, (ESBN)₂ClO₄, was obtained by the electrochemical oxidation of ESBN. The interaction between conduction electrons in the mixed-valence π-staking column of donor moieties and localized spins in the radical moieties in this salt was confirmed by the appearance of giant negative magnetoresistance (–70% at 2 K, 9 T). The spin-dependent electron transport in this system is derived from the intramolecular magnetic coupling by means of orthogonality of molecular orbitals.