The incidence of gastric cancer is extremely high in China, prompting the development of effective therapeutic strategies. Sodium selenite (SS) affects the proliferation and redifferentiation of gastric cancer cells and the Adriamycin prodrug Ac-Phe-Lys-PABC-ADM (PADM) reduces toxicity in gastric cancer treatment. However, the mechanisms involved therein remain unclear. In this study, nude mice were transplanted with SGC-7901 gastric cancer cells to construct a tumor xenograft model. After administration of SS and PADM, tumor weight and size were reduced. In addition, the levels of alanine aminotransferase, aspartate transaminase, creatinine, and lactate dehydrogenase were decreased, indicating improved hepatic and renal function and inhibited cancer cell metabolism. Furthermore, combined treatment of SS and PADM downregulated the expression of cell cycle-related proteins (cyclin-dependent kinase 4, Ki67, cyclin E, and cyclin D1), elevated that of proapoptosis proteins (Bax, cleaved caspase-3, cleaved caspase-9, and P53), and upregulated that of mitochondrial apoptosis-associated proteins (apoptotic protease activating factor 1 and second mitochondria-derived activator of caspases). In conclusion, combined treatment of SS and PADM effectively promoted apoptosis in gastric cancer xenografts via the mitochondrial apoptosis pathway.