Increasing studies have highlighted the critical role of lncRNAs in cancer pathogenesis and development. LncRNA maternally expressed gene 3 (MEG3) was reported to function as a tumor suppressor in breast cancer. However, the detailed molecular mechanism of MEG3 involved in breast cancer progression remains far from being addressed. Our findings showed that MEG3 was downregulated and miR-21 was upregulated in breast cancer patient tissues and cells. MEG3 overexpression suppressed cell proliferation and glycolysis, and induced apoptosis in breast cancer cells. MEG3 was demonstrated to function as a molecular sponge of miR-21 and suppress its expression. Moreover, miR-21 upregulation partially abolished the effects of MEG3 overexpression on cell proliferation, glycolysis, and apoptosis in breast cancer cells. Additionally, enforced expression of MEG3 reversed miR-21-mediated activation of PI3K/Akt pathway in breast cancer cells. In vivo experiment demonstrated that overexpression of MEG3 inhibited tumor growth in breast cancer by suppressing miR-21. In summary, MEG3 overexpression inhibited the tumorigenesis of breast cancer by downregulating miR-21 through the PI3K/Akt pathway.