A fluorescent cationic benzothiazole dye that selectively targets a G-quadruplex aptamer was designed and synthesized as a K⁺ sensor. The K⁺-driven aptamer sensor is based on the strategy of conformational transition from single-stranded DNA to G-quadruplex structure, leading to an amplified fluorescence signal in the reporter. This fluorescent sensor displayed high selectivity for K⁺, suggesting great potential for practical applications.