We previously established a thermodynamical model to calculate the specific frequencies of extremely low frequency-electromagnetic field (ELF-EMF) able to arrest the growth of cancer cells. In the present study, for the first time, we investigated the efficacy of this technology on osteosarcoma, and we applied a precise frequency of the electromagnetic field on three human osteosarcoma cell lines, grown as adherent cells and spheroids. We evaluated the antitumour efficacy of irradiation in terms of response to chemotherapeutic treatments, which is usually poor in this type of cancer. Importantly, the results of this novel combinatorial approach revealed that the specific exposure can potentiate the efficacy of several chemotherapeutic drugs, both on bidimensional and tridimensional cancer models. The effectiveness of cisplatinum, methotrexate, ifosfamide and doxorubicin was greatly increased by the concomitant application of the specific ELF-EMF. Moreover, our experiments confirmed that ELF-EMF inhibited the proliferation and modulated the mitochondrial metabolism of all cancer models tested, whereas mesenchymal cells were not affected. The latter finding is extremely valuable, given the importance of preserving the cell reservoir necessary for tissue regeneration after chemotherapy. Altogether, this novel evidence opens new avenues to the clinical applications of ELF-EMF in oncology.