Transition-metal complexes and organic radical molecules can be used to make electric conductors and ferromagnets(1-3), the optical properties of which can be controlled by changing temperature and are used as molecular switches and sensors(4-8). Whereas a number of organic radicals in solution show temperature-dependent optical properties(5,6,9-12), such behaviour in crystalline forms is more rare(13-15). Here, we show a fully reversible continuous thermochromism with a unique mechanism in purely organic crystals of diazaphenalenyl radical. This behaviour is based on changes in the diazaphenalenyl dimers coexisting in the crystal. From the X-ray crystal structure analyses and temperature-dependent visible spectra, we conclude the presence of a thermal equilibrium between sigma-bonded and pi-bonded dimers, which are separated by 2.62(6) kcal mol(-1). This conclusion is supported by room-temperature electron spin resonance spectra of the solid, which showed signals that are attributable to a thermally accessible triplet state of the pi-dimer structure. This proves the coexistence of two dimers of different bonding natures in the crystal, causing it to demonstrate thermometer-like behaviour.