We investigated the suitability of dithiocarbamate (DTC) capped Ag nanoparticles (NPs) as resonance light scattering (RLS) probes for the simultaneous sensing of Pb²⁺ and cysteine. The DTC capping ligands are generated by a very simple in situ method through reaction of carbon disulfide with diethanolamine as primary precursor molecules under ultrasonic irradiation. This strategy was based on the fact that Pb²⁺ could induce the aggregation of DTC–Ag NPs due to the strong metal affinity of DTC along with an enhanced RLS signal. After optimizing some experimental conditions (including the pH value of the solution, concentration of DTC–Ag NPs, and ion strength), a very simple and facile sensing system has been developed for the detection of Pb²⁺ in water based on RLS technology. The proposed system promises excellent selectivity, a wide linear response range and high sensitivity for Pb²⁺. The linear response range for Pb²⁺ was from 0.01 μM to 60 μM. The limit of detection (S/N = 3σ) for Pb²⁺ was as low as 4 nM. The proposed method was successfully used to detect Pb²⁺ in river and tap water samples, indicating the potential of this new, sensitive and selective method in water quality monitoring. Meanwhile, due to the strong binding preference of cysteine toward Pb²⁺ by the formation of Pb²⁺–S bonds, Pb²⁺ was removed from the surfaces of the DTC–Ag NPs, leading to redispersion of DTC–Ag NPs, along with a decreased RLS signal. The possibility of the proposed system for the sensing of cysteine was also investigated.