An efficient signal enhancement strategy for the detection of cysteine (Cys) was developed, which is based on the combination of a Cys-reduced reaction and the chelation of Al³⁺ with the Cys-promoted product. The Cys probe S-DPM was prepared by adding a 2,4-dinitrobenzenesulfonyl group onto a 2′-hydroxyldihydropyrimidobenzimidazole derivative (DPM). Upon treatment with Cys, S-DPM was reduced and the hydroxyl group was released in the product DPM, thereby enabling an excited-state intramolecular proton transfer (ESIPT) process. When Al³⁺ ions were added, the fluorescence signal was dramatically increased owing to the transformation of the DPM into DPM-Al³⁺ complexes with a very high fluorescence quantum yield (0.865). The proposed method allows the analysis of Cys by modulating the switch of the fluorescence of S-DPM with Cys ranging from 0.01 to 4.5 μM. The detection limit is 17 nM. As expected, the S-DPM probe was successfully applied for the determination of Cys in human serum and plasma samples with recoveries ranging from 97.2.0% to 105.5%. Fluorescence imaging experiments showed that S-DPM can track Cys in living cells. ESIPT compounds are frequently used for the construction of Cys probes, but this method measures the fluorescence of only a part of the Cys-reduced products. The use of S-DPM coupled with Al³⁺ overcomes this drawback.