To elucidate the causal associations between inflammatory bowel disease (IBD) and its subtypes—Crohn’s disease (CD) and ulcerative colitis (UC)—and breast cancer along with its subtypes—Estrogen Receptor-Positive (ER + BC) and Estrogen Receptor-Negative Breast Cancer (ER-BC), this study employs bidirectional two-sample Mendelian randomization analysis. This evidence concerns the gene ESR1 and ulcerative colitis.