A large first-order magnetic phase transition in an organic radical, 1,3,5-trilhia-2,4,6-triazapentalenyl is described, that occurs with a wide thermal hysteresis loop over the temperature range from 230 to 305 K. The high-temperature phase is paramagnetic, while the low-temperature phase is diamagnetic. The results reported here on room-temperature magnetic molecular bistability may have applications in thermal sensors, switching units and information storage media, based on organic radical crystals.