We report the results of a synchrotron-based high-resolution XPS study of the interaction of l-cysteine (Cys) with well-characterized colloidal gold nanoparticles (NPs, typical size 3-4 nm), which were pre-deposited on highly oriented pyrolytic graphite and then brought into contact with the aqueous solution of Cys by drop-casting. By comparison with data previously obtained for Cys deposition on flat Au substrates (single crystals and high quality films), we demonstrate the formation of a strong Cys/NP thiolate bond. The analysis of the line shape and adsorbate-induced Au 4f core level shift, backed by simulations of the NP structure, reveals the interaction of Cys with low-coordinated Au atoms belonging to the NP edge and corners. The analysis of the N 1s core-level indicates that neutral molecules are the most abundant species. The small facet size limits the formation of extended networks of zwitterionic molecules, typical of single crystal surfaces. This study provides a spectroscopic insight into the intense poisoning effect caused by a limited amount of Cys on Au catalysts described in previous reports.