Mere15, an anticancer polypeptide with a molecular weight of 15 kDa, is extracted from the marine species Meretrix meretrix. A previous study in our laboratory has confirmed that Mere15 displays a potent antitumor activity. However, the underlying mechanism of Mere15 still remains unclear. The effect of Mere15 on the growth of a variety of tumor cells was measured by the CCK-8 assay. Hoechst33342/PI double staining and flow cytometry assays were used to detect the apoptosis status of cancer cells. Western blotting was used to detect the expression of apoptosis-related proteins, migration and invasion-related protein, and the changes in the PI3K/Akt/mTOR signaling pathway-related proteins. Treatment with Mere15 inhibited cancer cell growth significantly. Scratch wound-healing assay, as well as Transwell experiments, revealed that the polypeptide was able to inhibit the invasion and migration of NSCLC cells significantly. Western blotting analysis confirmed that treatment with Mere15 inhibited the phosphorylation of PI3K, Akt, and mTOR significantly. The effects of Mere15 were also evaluated in the presence of an activator or inhibitor of the PI3K/Akt/mTOR pathway. Downregulated expression of MMP-2, MMP-9, and Snail, and increased expression of E-cadherin were also found in cells treated with Mere15. In vivo study revealed that Mere15 inhibited tumor growth significantly in xenograft nude mice bearing NCI-H460 cancer cells. The study provides evidence that Mere15 has the potential to be developed as a novel antimetastatic agent for the treatment of NSCLC patients. The work also provides further evidence that targeting PI3K/Akt/mTOR pathway is an important strategy for overcoming cancer metastasis.