Breast cancer treatment has advanced significantly, particularly for estrogen receptor-positive (ER+) tumors. Tamoxifen, an estrogen antagonist, is widely used; however, approximately 40% of patients develop resistance. Recent studies indicate that microRNAs, especially miR-155, play a critical role in this resistance. Our analysis of MCF-7 tamoxifen-sensitive (TAM-S) and tamoxifen-resistant (TAM-R) cells revealed that miR-155 is significantly upregulated in TAM-R cells. Overexpression of miR-155 in TAM-S cells increased resistance to tamoxifen. Additionally, genistein, a natural isoflavone from soybeans, effectively downregulated miR-155 and its targets associated with apoptosis and glucose metabolism, including STAT3 and hexokinase 2 (HK2). Notably, genistein also significantly decreased cell migration, suggesting potential anti-metastatic effects. Furthermore, genistein reduced glucose consumption, indicating its potential to overcome miR-155-mediated tamoxifen resistance and modulate the Warburg effect. These findings highlight genistein as a promising therapeutic agent for overcoming tamoxifen resistance in ER+ breast cancer and merit further investigation.