Gold nanoparticles (AuNPs) and AuNP-labelled antibodies can participate in the electrochemiluminescence (ECL) reaction of tris(2,2′-bipyridyl) ruthenium(II) (Ru(bpy)₃²⁺) as a co-reactant. The electrochemical and spectral characteristics of this new ECL system were confirmed by experiments, and its reaction mechanism was speculated to be different from that of typical ECL. The experimental results showed that the new ECL system exhibited higher ECL efficiency, and AuNP-labelled substances can be sensitively measured with the help of AuNPs. Based on the dual role of AuNPs as a co-reactant and marker, the AuNP–Ru(bpy)₃²⁺ ECL system will be a useful tool in biochemical analysis.