Organic neutral pi-monoradicals are promising semiconductors with balanced ambipolar carrier transport abilities, which arise from virtually identical spatial distribution of their singly occupied and unoccupied molecular orbitals, SOMO(alpha) and SOMO(beta), respectively. Herein, we disclose a boron-stabilized triphenylmethyl radical that shows outstanding thermal stability and resistance toward atmospheric conditions due to the substantial spin delocalization. The radical is used to fabricate organic Mott-insulator transistors that operate at room temperature, wherein the radical exhibits well-balanced ambipolar carrier transport properties.