© 2020 Orthopaedic Research Society. Published by Wiley Periodicals LLC Management of bone metastasis is becoming increasingly important. Thus, local and systemic treatment options have been developed for control. Although systemic administration of anticancer agents is effective for bone metastasis, it is often stopped because of poor general conditions or side effects. Therefore, it is highly desirable to develop a more effective and safer local treatment for bone metastasis. The purpose of the current study was to investigate the antitumor effects and safety of gelatin hydrogel microspheres incorporating cisplatin (GM-CDDP), which we developed as a sustained release system without harmful substances. First, we assessed GM-CDDP for its in vitro degradability and potential for sustained release. Second, in vivo antitumor and side effects were evaluated using a murine bone metastasis model of MDA-MB-231 human breast cancer cells incorporating GFP. In vitro, initial bursts were observed within 2 h and CDDP was released gradually with gelatin hydrogel degradation, which reached 100% at 48 h. In vivo, local administration of GM-CDDP (2 mg/kg) significantly suppressed tumor growth and bone osteolysis compared with the control, and local and systemic administration of free CDDP (2 mg/kg; p < 0.05). Local administration of GM-CDDP significantly reduced loss of body weight and elevation of blood urea nitrogen compared with the systemic administration of free CDDP (p <.05). The current study suggests that local administration of GM-CDDP achieves higher antitumor effects with a potential for lesser side effects compared with local or systemic administration of free CDDP.