The goal of this work was to clarify the effect of carbon-ion beams on reduction of the metastatic potential of malignant melanoma using in vitro and in vivo techniques. We utilized a 290 MeV/u carbon beam with a 6-cm spread-out Bragg-peak (SOBP), ¹³⁷Cs γ rays or 200 kVp X rays for irradiation, and in vitro murine melanoma B16/BL6 cells that were implanted into C57BL/6J mice. The metastatic abilities (migration, invasion and adhesion) were suppressed by carbon ion treatment at all doses that were tested, whereas invasion and migration tended to increase after X-ray irradiation at low dose. Biological effects of carbon ions increased with linear energy transfer (LET) for both cell killing and metastatic abilities, although the effects were more pronounced for migration and invasion. mRNA expression of E-cadherin was significantly downregulated with low-dose photon exposures, but increased with dose or LET. Expression of Mel-CAM and L1-CAM was upregulated after low-dose photon exposure, but decreased with dose, especially after carbon-ion treatment. Conversely, these molecules showed a reversal in expression changes, especially after low-dose photon exposure. Cell-cell adhesion may be an important contributor to the antimetastatic effect of carbon ion treatment. The number of lung metastases after local tumor irradiation significantly decreased with increased dose and LET, with carbon ions being more effective than γ rays. Integrating dose-response curves to examine the relationship between cell killing and lung metastasis clearly showed that carbon ions inhibit lung metastasis more efficiently than photons at the iso-effective level of cell killing. Thus, carbon ions were more effective than photon beams, not only at killing tumor cells, but also at inhibiting metastatic spread caused by tumor cells that survived irradiation.