Osteosarcoma is the most common primary malignant bone tumor and affects a significant portion of pediatric oncology patients. Although surgery and adjuvant chemotherapy confer significant survival benefits, many patients go on to develop metastatic disease, particularly to the lungs, secondary to development of drug resistance. Inhibition of protein phosphatase 2A with the small molecule, LB100, has demonstrated potent chemo- and radio-sensitizing properties in numerous pre-clinical tumor models. In this study, we showed that LB100 overcame DNA repair mechanisms in osteosarcoma cells treated with cisplatin, in vitro, and recapitulated these findings in an in vivo xenograft model. Notably, the addition of LB100 to cisplatin prevented development of pulmonary metastases in the majority of treated animals. Our data indicated the mechanism of chemo-sensitization by LB100 involved abrogation of the ATM/ATR-activated DNA damage response, leading to hyperphosphorylation of Chk proteins and persistent cyclin activity. In addition, LB100 exposure suppressed Akt signaling, leading to Mdm2-mediated proteasomal degradation of functional p53. Taken together, LB100 prevented repair of cisplatin-induced DNA damage, resulting in mitotic catastrophe and cell death.