Cortactin is a scaffolding protein that regulates Arp2/3-mediated actin polymerization. We showed in a previous study that cortactin was highly expressed in human stage II-III colorectal cancer (CRC) tissues. In the present study, using colony formation and CCK-8 assays, we showed that overexpression of cortactin accelerated the proliferation of CRC cells. Flow cytometric assays revealed that cortactin promoted G1/S phase cell cycle transition. Later, we constructed the phosphorylation mutation of cortactin at the Tyr421 residue. Colony formation and CCK-8 assays showed that cortactin/Tyr421A lost its ability to promote cell proliferation. Western blot analysis indicated that cortactin activated cyclin D1, but not cortactin/Tyr421A. Further study in nude mice revealed that there was a greater decrease in both tumor volume and tumor weight in animals injected with SW480/cortactin/Tyr421A cells than in those injected with SW480/cortactin/WT cells. Thus, the present study demonstrates that the cortactin Tyr421 residue is required to promote cell proliferation both in vitro and in vivo.