This work presents the enhanced electrochemiluminescence (ECL) of a newly prepared nanosilver–carbon nanodot (Ag–C-dot) composite and its application for the sensitive detection of sulfide (S²⁻) ions. The Ag–C-dot composite was easily prepared by adding silver nitrate into C-dot colloids through an alkaline reduction. The obtained Ag–C-dots were characterized by UV-vis spectra, fluorescence spectra and transmission electron microscopy. The electrochemical and ECL behaviors of the Ag–C-dot composite were investigated by cyclic voltammetry. Moreover, a simple label-free method to detect S²⁻ ions with a high selectivity and sensitivity has been developed based on the ECL of the Ag–C-dot composite in aqueous media. The sensing mechanism could be due to the strong and specific interaction between the S²⁻ ions and the Ag atoms/ions on the surface of the Ag–C-dot composite, which dramatically affects the resulting ECL of the Ag–C-dot composite. The linear response to detect S²⁻ ions ranges from 0.05 to 100 μM with a detection limit of 0.027 μM (∼1 ppb). This work indicates that the Ag–C-dot nanocomposite possesses potential applications for environment sensing.