Identification of bioactive natural products with anticancer activity as well as alleviating effects on chemotherapy-induced side effects has significant implications for cancer treatment. Betula platyphylla var. japonica, commonly known as Asian white birch, has been used in Chinese traditional medicine for a variety of purposes. In this study, the medicinal properties of betulin from B. platyphylla var. japonica useful for cancer management were investigated. LC/MS analysis revealed that betulin is a main chemical component of the EtOH extract of B. platyphylla var. japonica bark, and betulin was isolated from EtOH extract using an LC/MS-guided isolation method. Its structure was identified with 1H and 13C NMR spectroscopic data and LC/MS analysis and then compared to the previously reported spectroscopic and physical data. We first verified the cytotoxicity of betulin against three human lung adenocarcinoma cell lines, A549, H1264, and Calu-6, with IC50 values ranging from 18.7 to 39.6 μM. Regarding alleviation of side effects associated with anticancer chemotherapy, betulin ameliorated cisplatin-induced renal cell damage to 80% of the control value from the concentration of 5 μM. In addition, betulin showed anti-gastritis activity against ethanol-induced gastric damage in rats and notably reduced the gastric damage index compared to control in a concentration-dependent manner. These findings provide the first experimental evidence for potential use of B. platyphylla var. japonica as a functional food for cancer treatment that simultaneously alleviates the side effects of chemotherapy.