Since abnormal levels of hydrogen sulphide (H2S) correlate with various diseases, simple methods for its rapid and sensitive detection are highly required. Herein, we introduce a new electrochemiluminescent probe 1 for H2S based on a cyclometalated iridium(iii) complex. o-(Azidomethyl)benzoate ester groups on the main ligands of probe 1 react selectively with H2S, resulting in cascade reactions involving H2S-mediated reduction and intramolecular cyclization/ester cleavage. With this structural change induced by H2S, the intrinsic electrochemiluminescence (ECL) of 1 decreased greatly due to the unfavourable electron transfer of a tripropylamine (TPA) radical. Probe 1 showed a high ECL turn-off ratio and good selectivity for H2S over various anions and biothiols. The sensing mechanism of H2S was elucidated using 1H NMR spectroscopy and MALDI-TOF mass spectrometry analyses.