Cathepsin S (CatS) is a lysosomal enzyme belonging to the papain family of cysteine proteases. Studies have demonstrated that CatS plays a pivotal role in antigen presentation and is associated with Alzheimer's disease, atherosclerosis, and obesity, positioning it as a potential target in related disorders. Consequently, the development of a selective and straightforward method for detecting CatS is crucial. In this study, we introduce a colorimetric approach utilizing gold nanoparticles (AuNPs) and CatS-specific peptides for the detection of CatS activity. The CatS enzyme cleavage peptides, PMGAPC or PMGAPC-NH2, can stabilize AuNPs in a hydrosaline solution without aggregation. Upon hydrolysis of PMGAPC or PMGAPC-NH2 by CatS, the AuNPs aggregate with a color change from red to blue. After optimizing the size of the AuNPs and the concentration of the peptides, the limits of detection for CatS were both to be 10 nM, when PMGAPC or PMGAPC-NH2 were used to protect the AuNPs. Therefore, this colorimetric assay, which employs peptides and AuNPs, enables not only a selective but also a simple detection of CatS, visible to the naked eye.