Methionine dependency describes the characteristic rapid in vitro death of most tumor cells in the absence of methionine. Combining chemotherapy with dietary methionine deprivation [methionine-deficient diet (MDD)] at tolerable levels has vast potential in tumor treatment; however, it is limited by MDD-induced toxicity during extended deprivation. Recent advances in imaging and irradiation delivery have created the field of stereotactic body radiotherapy (SBRT), where fewer large-dose fractions delivered in less time result in increased local-tumor control, which could be maximally synergistic with an MDD short course. Identification of the lowest effective methionine dietary intake not associated with toxicity will further enhance the cancer therapy potential. In this study, we investigated the effects of MDD and methionine-restricted diet (MRD) in primary and metastatic melanoma models in combination with radiotherapy (RT). In vitro, MDD dose-dependently sensitized mouse and human melanoma cell lines to RT. In vivo in mice, MDD substantially potentiated the effects of RT by a significant delay in tumor growth, in comparison with administering MDD or RT alone. The antitumor effects of an MDD/RT approach were due to effects on one-carbon metabolism, resulting in impaired methionine biotransformation via downregulation of Mat2a, which encodes methionine adenosyltransferase 2A. Furthermore, and probably most importantly, MDD and MRD substantially diminished metastatic potential; the antitumor MRD effects were not associated with toxicity to normal tissue. Our findings suggest that modulation of methionine intake holds substantial promise for use with short-course SBRT for cancer treatment.