Context: Forkhead boxM1 (FoxM1) transcription factor has been shown to promote pathogenesis of several malignancies. FoxM1 has also been shown to be associated with matrix metalloproteinases (MMP) in various cancers. However, little is known about its function in papillary thyroid carcinoma (PTC). Objective: In this study, we investigated the role of FoxM1 in pathogenesis in a large series of PTC in a tissue microarray format followed by in vitro and in vivo studies using PTC cell lines and nude mice. Design: Expression of FoxM1 and its associated proteins were investigated in Middle Eastern PTC samples by immunohistochemistry. Apoptosis was measured by flow cytometry and immunoblotting. Invasion and migration studies were performed using 8-μm Transwell plates. Results: FoxM1 was overexpressed in 28.4% of PTC and significantly associated with activated matrix metalloproteinase-9 (MMP-9) (P = 0.0004), X-linked inhibitor of apoptosis protein (XIAP) (P = 0.0024), and B-cell lymphoma-extra large (Bcl-XL) (P = 0.0014) expression. Treatment of PTC cell lines with thiostrepton, an inhibitor of FoxM1, resulted in inhibition of cell viability via induction of apoptosis. In addition, thiostrepton treatment of PTC cells or expression of FoxM1-specific small interfering RNA down-regulated expression of FoxM1 accompanied with decreased MMP-2 and MMP-9 expression. Furthermore, inhibition of FoxM1 attenuated migration and invasion of PTC cells. Interestingly, overexpression of FoxM1 rescued the effects of thiostrepton in PTC cell lines. Finally, treatment of PTC cell line xenografts with thiostrepton resulted in growth inhibition of tumors in nude mice via down-regulation of FoxM1 and MMP-9 and MMP-2. Conclusion: Altogether, this is the first study showing that FoxM1 and its associated signaling pathway play a critical role in the pathogenesis of PTC and may be a potential target for therapeutic intervention for treatment of these cancers.