In this article, we demonstrate that the synthetic cannabinoid R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)-(1-naphthalenyl) methanone mesylate (WIN 55,212-2) sensitizes human hepatocellular carcinoma (HCC) cells to apoptosis mediated by tumor necrosis-related apoptosis inducing ligand (TRAIL). The apoptotic mechanism induced by treatment with WIN/TRAIL combination involved the loss of the mitochondrial transmembrane potential and led to the activation of caspases. In HCC cells, WIN treatment induced the up-regulation of TRAIL death receptor DR5, an effect that seemed to be related to the increase in the level of p8 and CHOP, two factors implicated in cellular stress response and apoptosis. This relationship was suggested by the observation that the down-regulation of p8 or CHOP by specific small interfering RNAs attenuated both WIN-mediated DR5 up-regulation and the cytotoxicity induced by WIN/TRAIL cotreatment. Moreover, WIN induced a significant decrease in the levels of some survival factors (survivin, c-inhibitor of apoptosis protein 2, and Bcl-2) and in particular in that of the active phosphorylated form of AKT. This event seemed to be dependent on the transcription factor peroxisome proliferator-activated receptor-gamma whose level significantly increased after WIN treatment. Therefore, both the induction of DR5 via p8 and CHOP and the down-regulation of survival factors seem to be crucial for the marked synergistic effects induced by the two drugs in HCC cells. Taken together, the results reported in this article indicate that WIN/TRAIL combination could represent a novel important tool for the treatment of HCC.