Mantle cell lymphoma (MCL) is an aggressive B-cell non-Hodgkin lymphoma characterized by the t(11;14)(q13;q32) cytogenetic abnormality and cyclin D1 overexpression. We have found evidence that Forkhead box M1 (FOXM1), a transcription factor with oncogenic potential, is important in the pathogenesis of MCL. Relatively high levels of FOXM1 proteins were detectable in all six MCL cell lines examined. By immunohistochemistry, we consistently found a subset of FOXM1-positive cells in MCL tumors. Analysis of two Gene Expression Omnibus (GEO) datasets from MCL patients showed that elevated FOXM1 levels significantly correlate with a worse clinical outcome. In MCL cell lines, inhibition of FOXM1 using thiostrepton or shRNA effectively triggered apoptosis and significantly reduced cell growth. FOXM1 forms a positive feedback loop with NFκB in MCL cells. Specifically, inhibition of FOXM1 dramatically decreased the protein level/transcription activity of p65, while enforced FOXM1 expression upregulated p65 and downregulated IκBα, a key NFκB inhibitor. Conversely, curcumin-mediated NFκB inhibition decreased the protein level/DNA binding of FOXM1, while transduction of a constitutively active IKKα construct into MCL cells significantly dampened the inhibitory effects of thiostrepton. Confocal microscopy revealed that FOXM1 and p65 colocalize with each other. In conclusion, FOXM1 and NFκB work collaboratively in promoting the growth and drug resistance of MCL, and FOXM1 may be a potentially useful therapeutic target.