KIOM-C, a novel herbal formula, was recently reported to be effective for treating pigs suffering from porcine circovirus-associated disease (PCVAD). In addition, administration of KIOM-C promoted clearance of influenza virus via production of antiviral cytokines, such as TNF-α and IFN-γ. Since metastasis is the major cause of cancer-related death and the greatest challenge in cancer treatment, we investigated the effect of KIOM-C on the metastatic potential of HT1080 and B16F10 cells. We observed inhibitory properties of KIOM-C in colony-forming activity, migration and invasion. Matrix metalloproteinase-9 (MMP-9) activity in the resting and PMA-stimulated state in HT1080 cells was dose-dependently decreased by KIOM-C treatment via suppression of NF-κB activation. In addition, daily oral administration of KIOM-C at doses of 170 and 510 mg/kg, the corresponding human adult daily doses, efficiently blocked lung metastasis in C57BL/6J mice following injection of B16F10 cells in the tail veins. In particular, none of the mice administered KIOM-C during the experimental period exhibited systemic toxicity, such as body weight loss or liver and kidney dysfunction. Collectively, our results suggest that KIOM-C is a potential therapeutic formula useful as a safe herbal medicine for controlling metastatic cancer.