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Quantum dots (QDs) are intensively studied and developed for the detection of toxic heavy metal ions, notably Cd²⁺. However, a severe drawback is that the probing QDs themselves often are based on highly toxic elements, such as Pb or Cd. Here, we report on a one-step aqueous synthesis of more benign and hydrophilic AgInZnS QDs using 3-mercaptopropionic acid as a ligand with a high photoluminescence quantum yield of up to 41%, which were successfully employed as a fluorescent probe with a turn-on mode for detection of Cd²⁺ in aqueous solutions. Specifically, we determine that the effective detection range of Cd²⁺ in aqueous solution is 0.1–290 μM, with the lower limit of detection being 37.8 nM. We further establish that the excellent turn-on detection of Cd²⁺ is due to that surface defects on the AgInZnS QDs are effectively passivated by the Cd²⁺, as verified by a prolonged fluorescent lifetime and an increased photoluminescence quantum yield. We finally demonstrate that the AgInZnS QD probe is capable of detecting Cd²⁺ in lake water samples, and that it meets the WHO standard.