Curcumin has been shown to have potent anticancer activities like inhibition of cell proliferation, induction of apoptosis, and suppression of angiogenesis. Transforming growth factor-beta (TGF-beta) signaling plays a complex role in tumor suppression and promotion depending on the tumor type and stage. However, the effect of curcumin on TGF-beta signaling in cancer cells and the role of TGF-beta signaling in curcumin-induced anticancer activities have not been determined. Here, we investigate the role of curcumin on TGF-beta signaling, and whether TGF-beta signaling is involved in the antitumor activities of curcumin. Human non-small cell lung cancer (NSCLC) cell lines, ACC-LC-176 (without TGF-beta signaling), H358, and A549 (with TGF-beta signaling) were treated with curcumin to determine cell growth, apoptosis, and tumorigenicity. Antitumor activities of curcumin were determined using these cell lines and an in vivo mouse model. We also tested the effect of curcumin on TGF-beta/Smad signaling by western blotting and by luciferase assays. Curcumin inhibited cell growth and induced apoptosis of all three NSCLC cell lines in vitro and in vivo. It significantly reduced subcutaneous tumor growth by these three cell lines irrespective of TGF-beta signaling status. Curcumin inhibited TGF-beta-induced Smad2/3 phosphorylation and transcription in H358 and A549 cells, but not in ACC-LC-176 cells. Curcumin reduces tumorigenicity of human lung cancer cells in vitro and in vivo by inhibiting cell proliferation and promoting apoptosis. These results suggest that TGF-beta signaling is not directly involved in curcumin-mediated growth inhibition, induction of apoptosis, and inhibition of tumorigenicity.