Cysteine cathepsins are lysosomal enzymes that belong to the papain family and can induce the degradation of damaged proteins through the endo-lysosomal pathway. It is highly upregulated in many cancers by regulating gene amplification and transcriptional, translational, and post-transcriptional modifications. Cathepsin S is part of the cysteine cathepsin family. Many studies have demonstrated that cathepsin S not only plays a specific role in MHC class II antigen presentation but also plays a crucial role in cancers. Cathepsin S is more stable at a neutral pH compared to other cysteine cathepsins, which supports the importance of cathepsin S in disease microenvironments. Therefore, the dysregulation of cathepsin S has participated in a variety of pathological processes, including cancer, arthritis, and cardiovascular disease. Furthermore, a decrease or depletion in the expression of cathepsin S has been implicated in the processes of tumor growth, invasion, metastasis, and angiogenesis. Taken together, cathepsin S has been suggested as an attractive therapeutic target for cancer therapy. In this review, the known involvement of cathepsin S in diseases, particularly with respect to recent work indicating its role in cancer therapy, is examined. An overview of current literature on the inhibitors of cathepsin S as a therapeutic target for cancer is also provided.