Transforming growth factor-β (TGF-β) signaling pathways are well-recognized for their role in proliferation and epithelial-mesenchymal transition (EMT) of cancer cells, but much less is understood about their contribution to interactions with other signaling events. Recent studies have indicated that crosstalk between TGF-β and Ras signaling makes a contribution to TGF-β-mediated EMT. Here, we demonstrate that Jumonji domain containing-3 (JMJD3 also called KDM6B) promotes TGF-β-mediated Smad activation and EMT in Ras-activated lung cancer cells. JMJD3 in lung cancer patients was significantly increased and JMJD3 expression in lung tumor tissues was correlated with expression of K-Ras or H-Ras in particular, and its expression was regulated by Ras activity in lung cancer cells. JMJD3 promotes TGF-β-induced Smad activation and EMT in Ras-activated lung cancer cells through the induction of syntenin, a protein that regulates TGF-β receptor activation upon ligand binding. Tissue array and ChIP analysis revealed that JMJD3 epigenetically induces syntenin expression by directly regulating H3K27 methylation levels. Mechanical exploration identified a physical and functional association of JMJD3 with syntenin presiding over the TGF-β in Ras-activated lung cancer cells. Taken together, these findings provide new insight into the mechanisms by which JMJD3 promotes syntenin expression resulting in oncogenic Ras cooperation with TGF-β to promote EMT.