Midkine (MDK), a heparin-binding growth factor, modulates the proliferation and migration of various cells, is often highly expressed in many malignant tumors, and may act as an oncoprotein. We found that MDK is overexpressed in clinical human gastric cancer tissues relative to its expression in adjacent noncancerous tissues. To further investigate the biological activities of MDK in gastric cancer, we introduced the MDK gene into human SGC7901 gastric cancer cells, where it contributed to the proliferation of SGC7901 cells in vitro and in vivo. Conversely, the knockdown of MDK expression by siRNA resulted in significantly reduced proliferation of BGC823 cells. Our study also shows that MDK activates both the Akt and ERK1/2 pathways and upregulates the expression of several cell-cycle-related proteins, including cyclin A, cyclin D1, Cdk2, Cdk4, and Cdk6, which in part explains the contribution of MDK to gastric cancer cell survival and growth. These results demonstrate that MDK contributes to gastric cancer cell proliferation and suggest that it plays an important role in the development of human gastric cancer.