The selective ratiometric red-emission detection of Hg²⁺ ions in aqueous buffered solutions and live cells is still a significant challenge. In the present study, we synthesized a fluorescent probe (1) based on an unnatural peptide receptor containing sulfonamide groups with an aggregation-induced emission and twisted internal charge transfer (TICT)-active fluorophore, cyanostilbene. 1 exhibited a highly selective ratiometric response to Hg²⁺ among 14 metal ions tested by ratiometric red-emission at 600 nm, with a clear isoemissive point in purely aqueous solution containing 1% DMSO. The ratiometric response for Hg²⁺ ions was complete within 3 min and the ratiometric responses induced by Hg²⁺ ions did not suffer considerable interference from the other metal ions. The ratiometric response was complete for less than 7 μM Hg²⁺ and 1 had a potent binding affinity (7.42 × 10⁻⁶ M, R² = 0.98) for Hg²⁺ and a nanomolar detection limit. 1 detected Hg²⁺ ions by ratiometric responses in aqueous buffered solutions over a wide range of pH (5.5–11.5). Binding mode studies using TEM, NMR, IR, and a mass spectrometer revealed that the sulfonamide groups of the unnatural peptide receptor played an important role in the complexation of Hg²⁺ and in the complexation-induced nano-sized aggregates, which resulted in a significant increase in emissions at 600 nm and a decrease in emissions at 535 nm. 1 quantified micro-molar concentrations (0–6 μM) of Hg²⁺ in tap water and groundwater by ratiometric detection. Furthermore, 1 passed through the lipid membranes of live cells and detected intracellular Hg²⁺ ions at 2 μM by a ratiometric red-emission change.