To improve conventional chemotherapeutic efficacy, a combination use of traditional medicines is effective but detailed mechanisms have been rarely elucidated. In the this study, we attempted to clarify how triptolide (PG490), an oxygenated diterpene derived from a Chinese herb, enhances the cisplatin (CDDP)-induced cytotoxicity in urothelial cancer cells. Our results showed that a combined CDDP/triptolide therapy induced apoptosis in urothelial cancer cell lines with wild-type p53, but not in those with mutant-type p53 or normal human urothelium. As the mechanism, triptolide suppressed CDDP-induced p53 transcriptional activity, leading to p21 attenuation, which promoted apoptosis via the activation of c-Jun N-terminal kinase (JNK) and Bax. We further demonstrated that the functional regulation of p53 by triptolide was mediated by an intranuclear association of p53 with glycogen synthase kinase-3beta (GSK3beta), which was inactivated by protein kinase C (PKC). This modulation of the PKC-GSK3beta axis by triptolide was observed in a cancer-specific manner. A mouse xenograft model also showed that a combined CDDP/triptolide therapy completely suppressed tumor growth without any side effects. We expect that cancer-specific enhancement of CDDP-induced cytotoxicity with triptolide may effectively overcome the resistance to a CDDP-based conventional chemotherapy as a treatment for urothelial cancer.