A novel ultrasensitive sensing system for the rapid detection of cytochrome c (Cyt C) was developed on the basis of an electrochemiluminescence (ECL) method. A nanocomposite biosensor was made of reduced graphene oxide decorated with cerium oxide/tris(2,2-bipyridyl)ruthenium(II)/chitosan (CeO2NPs-RGO/ Ru(bpy)3 (2+)/CHIT) and used for this purpose. The ECL signal was produced by an electrochemical interaction between Ru(bpy)3 (2+) and tripropyl amine (TPA) on the surface of the electrode. Addition of Cyt C to the solution decreases the ECL signal due to its affinity for TPA and inhibition of its reaction with Ru(bpy)3 (2+). The effects of the amount of CeO2NPs-RGO, Ru(bpy)3 (2+), TPA concentration as a co-reactant, and the pH of the electrolyte solution on the ECL signal intensity were studied and optimized. The results showed that the method was fast, reproducible, sensitive, and stable for the detection of Cyt C. The method has a linear range from 2.5 nM to 2 μM (R (2) = 0.995) with a detection limit of 0.7 nM. Finally, the proposed biosensor was used for the determination of Cyt C in human serum samples with RSDs of 1.8-3.6 %. The results demonstrate that this solid-state ECL quenching biosensor has high sensitivity, selectivity, and good stability. Graphical Abstract A novel solid-state electrochemiluminescence sensor for detection of cytochrome C based on Ceria Nanoparticles Decorated Reduced Graphene Oxide Nanocomposite.