In this work, we report a highly sensitive fluorescent sensor (TPE-QN) with aggregation-induced emission (AIE) characteristics for the detection of iodide (I⁻) and mercury (Hg²⁺) ions. TPE-QN is based on tetraphenylethene (TPE)-functionalized quinolinium salts with hexafluorophosphate (PF₆⁻) as the counterion, and exhibits intense red emission in aqueous media when nanoaggregates are formed. In the presence of I⁻, the emission of TPE-QN can be effectively quenched due to synergistic electrostatic interactions and drastic collisions between aggregates of TPE-QN and I⁻, enabling TPE-QN to work as a fluorescent “turn-off” sensor for I⁻ with a detection limit of 22.6 nM. Moreover, the complex of TPE-QN and I⁻ (TPE-QN–I) can secondarily recognize Hg²⁺ by showing a fluorescence “turn-on” signal because of the high affinity between Hg²⁺ and I⁻, and the detection limit for Hg²⁺ is as low as 71.8 nM. Furthermore, the high selectivity and sensitivity of TPE-QN makes it quite qualified for detecting the low concentration of I⁻ and Hg²⁺ in real samples such as running water and urine.