The therapeutic efficacy of bone tumor treatment is primarily limited by inadequate tumor resection, resulting in recurrence and metastasis, as well as the deep location of tumors. Herein, an injectable doxorubicin (DOX)-loaded magnetic alginate hydrogel (DOX@MAH) was developed to evaluate the efficacy of an alternating magnetic field (AMF)-responsive, chemothermal synergistic therapy for multimodality treatment of bone tumors. The prepared hydrogel exhibits a superior drug-loading capacity and a continuous DOX release. This multifunctionality can be attributed to the combined use of DOX for chemotherapy and iron oxide nanoparticle-containing alginate hydrogels as magnetic hyperthermia agents to generate hyperthermia for tumor elimination without the limit on penetration depth. Moreover, the hydrogel can be formed when in contact with the calcium ions, which are abundant in bone tissues; therefore, this hydrogel could perfectly fit the bone defects caused by the surgical removal of the bone tumor tissue, and the hydrogel could tightly attach the surgical margin of the bone to realize a high efficacy residual tumor tissue elimination treated by chemothermal synergistic therapy. The hydrogel demonstrates excellent hyperthermia performance, as evidenced by cytotoxicity tests on tumor cells. These tests reveal that the combined therapy based on DOX@MAH under AMF significantly induces cell death compared to single magnetic hyperthermia or chemotherapy. antitumor effects in tumor-bearing mice demonstrate that DOX@MAH injection at the tumor site effectively inhibits tumor growth and leads to tumor necrosis. This work not only establishes an effective DOX@MAH system as a synergistic chemothermal therapy platform for treating bone tumors but also sheds light on the application of alginate to combine calcium ions of the bone to treat bone defect diseases.