Chemical reduction of [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethylamino)ethene (3) leads to the [3]2+[12]2– salt (4) whose X-ray crystal structure reveals diamagnetic π-dimers of the [1,2,5]thiadiazolo[3,4-c][1,2,5]-thiadiazolidyl radical anion (1). The interplanar separation of 324.5 pm in the [12]2– dimers is significantly shorter than the sum of the isotropic van der Waals radii of two S atoms of 348–358 pm. CASSCF calculations suggest that the largest contributor to the singlet ground state of these π-dimers is the closed-shell electronic configuration, although a noticeable contribution from a configuration with a singlet biradical character was also found.