Montelukast is an anti-asthmatic medication, and has recently showed its inhibitory effects on the proliferation of cancers. The purpose of this study was to identify the cytotoxic effects of montelukast on multiple myeloma (MM) cells and the combination effects of montelukast and carfilzomib in the treatment of MM. Results revealed that montelukast induced a dose- and time-dependent cytotoxicity in MM cells lines and significantly suppressed the colony formation of myeloma cells. Furthermore, montelukast enhanced the cytotoxicity of carfilzomib in MM cell lines. This anti-tumor effect was associated with decreased c-Myc via the inhibition of mTOR signaling pathway. Moreover, the combination of montelukast and carfilzomib induced apoptosis of myeloma cells effectively, even in the presence of bone marrow stromal cells (BMSCs). It is more important to note that the co-treatment exhibited similar cytocidal effects in carfilzomib-resistant cell lines (U266R and 8226R). In addition, the combined effects were noted in two MM xenograft mice models and 7 cases of human CD138(+) myeloma cells (4 newly diagnosed cases and 3 relapsed cases) with no cytotoxicity on peripheral blood mononuclear cells (PBMCs) from 5 healthy donors. Our data suggested that montelukast enhanced the cytotoxicity of carfilzomib in both carfilzomib-sensitive and carfilzomib-resistant MM cell lines. These findings may facilitate the development of therapeutic strategies and provide a promising therapeutic combination regimen for the treatment of refractory myeloma.