Monocyte chemoattractant protein-1 (MCP-1) is a chemotactic cytokine that can bind to its receptor cysteine-cysteine chemokine receptor 2 (CCR2) and plays an important role in breast cancer cell metastasis. However, the molecular mechanisms underlying MCP-1-induced alterations in cellular functions during tumor progression are poorly understood. Here, we showed that MCP-1 stimulated the epithelial-mesenchymal transition (EMT) and induced the tumorigenesis of breast cancer cells by downregulating E-cadherin, upregulating vimentin and fibronectin, activating matrix metallopeptidase-2 (MMP-2), and promoting migration and invasion. Moreover, MCP-1 treatment reduced glycogen synthase kinase-3β (GSK-3β) activity via the MEK/ERK-mediated phosphorylation of serine-9 in MCF-7 cells. The inhibition of MEK/ERK by U0126 attenuated the MCP-1-induced phosphorylation of GSK-3β and decreased the expression of Snail, an EMT-related transcription factor, leading to the inhibition of MCF-7 cell migration and invasion. Inactivation of GSK-3β by LiCl (lithium chloride) treatment notably increased MMP-2 activity, vascular endothelial growth factor expression and EMT of MCF-7 cells. These findings revealed that MCP-1-induced EMT and migration are mediated by the ERK/GSK-3β/Snail pathway, and identified a potential novel target for therapeutic intervention in breast cancer.