Thymoquinone (TQ) combined with Cisplatin may augment its anticancer effect on hepatocellular carcinoma (HCC), through oxidative stress mitigation and endoplasmic reticulum (ER) protein modulation. Fifty adult male Wistar albino rats were assigned into five equal experimental groups (n = 10); 1) Control, 2) diethylnitrosamine/carbon tetrachloride-induced liver tumorigenesis model (HCC), 3) Cisplatin (2 mg.kg-1ip) treated rats, 4) Thymoquinone treated group (20 mg.kg-1oral), and 5) group treated with both drugs as in Groups 3 and 4. Treatment regimens started following model confirmation and continued for 4 weeks. In the HCC model, we detected elevated ER chaperone glucose-regulated protein-78 (GRP78) and reduced C/EBP-homologous protein (CHOP)-mediated apoptosis that was accompanied by the elevated alpha-fetoprotein (AFP) marker and deteriorated liver functions. Our original results indicated that Thymoquinone potentiated the pro-apoptotic effect of cisplatin by modulating GRP78/CHOP signaling. Cisplatin/TQ reduced the elevated GRP78 and induced CHOP-mediated apoptosis in the diseased liver tissues compared to the HCC and Cisplatin treated groups. Cisplatin/TQ combination normalized AFP levels and improved liver functions compared to both HCC and cisplatin groups alone. In conclusion, Thymoquinone enhanced the efficacy of Cisplatin in HCC treatment by modulating the GRP78/CHOP/caspase-3 pathway. Thymoquinone is recommended to achieve greater therapeutic benefits and reduce the cisplatin hepatotoxicity in HCC management.