We studied the single‐molecule conductance through an acid oxidant triggered phenothiazine (PTZ‐) based radical junction using the mechanically controllable break junction technique. The electrical conductance of the radical state was enhanced by up to 200 times compared to the neutral state, with high stability lasting for at least two months and high junction formation probability at room‐temperature. Theoretical studies revealed that the conductance increase is due to a significant decrease of the HOMO–LUMO gap and also the enhanced transmission close to the HOMO orbital when the radical forms. The large conductance enhancement induced by the formation of the stable PTZ radical molecule will lead to promising applications in single‐molecule electronics and spintronics. Single‐molecule conductances of phenothiazine radicals have been measured using the mechanically controllable break junction technique. Triggered by an acid oxidant, the radical cation of phenothiazine enhances the charge‐transport property by up to 200 times with high stability and high junction formation probability at room temperature, which leads to promising applications in single‐molecule electronics and spintronics.