In studies aimed at developing new organic spin materials, we prepared a triisopropylsilylethynyl substituted tri(9-anthryl)methyl (TAntM) radical. The TIPS-ethynyl group in this radical effectively suppresses its reactivity, resulting in extremely high stability in air for at least one month. Chemical modification of the radical using [4+2] Diels-Alder reaction proceeds even at room temperature. Because harsh conditions and metal-catalyzed reactions are not required, this post-modification strategy should be highly versatile for use in constructing unique spin-labelled molecules.