Nuclear factor kappa B (NF-κB) signaling pathway is activated in many colorectal cancer (CRC) cells and in the tumor microenvironment, which plays a critical role in cancer initiation, development, and response to therapies. In the present study, we found that the widely used antimalarial drug mefloquine was a NF-κB inhibitor that blocked the activation of IκBα kinase, leading to reduction of IκBα degradation, decrease of p65 phosphorylation, and suppressed expression of NF-κB target genes in CRC cells. We also found that mefloquine induced growth arrest and apoptosis of CRC cells harboring phosphorylated p65 in culture and in mice. Furthermore, expression of constitutive active IKKβ kinase significantly attenuated the cytotoxic effect of the compound. These results showed that mefloquine could exert antitumor action through inhibiting the NF-κB signaling pathway, and indicated that the antimalarial drug might be repurposed for anti-CRC therapy in the clinic as a single agent or in combination with other anticancer drugs.