A highly sensitive photoelectrochemical (PEC) cysteine sensor was fabricated based on reduced graphene oxide (RGO)/CdS:Mn nanocomposites. The RGO/CdS:Mn nanocomposites were synthesized by a facile, one-step method in aqueous solution. The doped Mn ions could trap electrons, improve the electron-hole pairs' life, and enhance the visible light absorption intensity, which are all beneficial for the enhancement of the PEC performance. In addition, the RGO/CdS:Mn nanocomposites inherited the excellent electron transport property of graphene, and facilitated the spatial separation of charge carrier, therefore result in the enhanced photocurrent intensity, making it a promising candidate for PEC application. Based on the enhanced PEC performance, cysteine was chosen as a model molecular to fabricate the PEC sensor. The sensor displayed excellent analytical performance for the detection of cysteine with a broad linear range from 0.11 mu M to 20 mM and a low detection limit of 0.02 mu M. The as-synthesized RGO/CdS:Mn nanocomposites exhibited obviously enhanced PEC performance, which will be a promising candidate for PEC sensor and other photoelectric devices. (C) 2015 Elsevier B.V. All rights reserved.