A novel phenothiazine-based derivative was designed and synthesized in this work, this molecule exhibited an obvious fluorescence quenching upon addition of Hg²⁺ due to the formation of a 1 : 1 metal–ligand complex. In addition to the high sensitivity toward Hg(II), it also displayed a high selectivity for Hg²⁺ over other transition metal ions in a rapid response time (<10 s). Simultaneously, the cell imaging experiment demonstrated the value of the sensor in fluorescent visualization of Hg²⁺ in biological systems.