Transforming growth factor-β1 (TGF-β1) signaling and high mobility group A (HMGA1) are known to play essential roles in the progression of breast cancer by inducing epithelial-mesenchymal transition. However, the correlation between TGF-β1 and HMGA1 in breast cancer cell is not yet well understood. In this study, we determined the effects of TGF-β1 on HMGA1 expression in breast cancer cells and examined the role of HMGA1 in breast cancer progression. Our results demonstrated that TGF-β1 induced the expression of HMGA1 in both MCF-7 and MDA-MB-231 breast cancer cells, as shown by RT-qPCR and immunofluorescence staining; however, the TGF-β1-induced expression of HMGA was blocked by treatment of the cells with phosphatidylinositol-3 kinase (PI3K) signaling inhibitors. Moreover, the HMGA1 promoter activity was found to be activated by TGF-β1 in the MCF-7 and MDA-MB-231 cells and we found that specificity protein 1 (Sp1) was involved in the TGF-β1-induced HMGA1 promoter activity, as shown by luciferase activity assay. Furthermore, the enforced expression of HMGA1 by transfection with a HMGA1 promoter enhanced cellular oncogenic properties, including proliferation, migration and invasion, and a tissue microarray revealed that breast tumors expressing human epidermal growth factor receptor 2 (HER2) showed higher expression levels of HMGA1 (P=0.007). In addition, higher HMGA1 expression levels were also observed in the ductal breast cancer cases compared with the lobular breast cancer cases (P=0.000). These findings establish the first link between HMGA1 and TGF-β1 in breast cancer, providing further evidence of the pivotal role of HMGA1 in breast cancer progression.