The function of cellular prion protein (PrP^C), the essential protein for the pathogenesis and transmission of prion diseases, is still largely unknown. The putative roles of PrP^C are thought to be related to cell signaling, survival, and differentiation. In a previous study, we showed that PrP^C was overexpressed in gastric cancer tissues. In the present report, we show that ectopic expression of PrP^C could promote tumorigenesis, proliferation, and G1/S transition in gastric cancer cells. Furthermore, CyclinD1, a protein related to cell cycle, was shown to be significantly up-regulated by PrP^C at both mRNA and protein levels. PI3K/Akt pathway mediated above PrP^C signal since PrP^C increased the expression of phosphorylated Akt, and the specific inhibitor of Akt, LY294002, could markedly suppress growth of SGC7901 and transactivation of CyclinD1 induced by PrP^C. Octapeptide repeat region played a vital role in this function, as deletion of this region abolished or reduced these effects. Collectively, this study demonstrates that overexpression of PrP^C might promote the tumorigenesis and proliferation of gastric cancer cells at least partially through activation of PI3K/Akt pathway and subsequent transcriptional activation of CyclinD1 to regulate the G1/S phase transition, in which octapeptide repeat region might be an indispensable region.--Liang, J., Pan, Y., Zhang, D., Guo, C., Shi, Y., Wang, J., Chen, Y., Wang, X., Liu, J., Guo, X., Chen, Z., Qiao, T., Fan, D. Cellular prion protein promotes proliferation and G1/S transition of human gastric cancer cells SGC7901 and AGS.