A clear bidirectional relationship exists between malignant cancers including breast cancer and different autoimmune diseases. However, none of the studies have assessed whether this association is causal and none have used Mendelian randomization for determining this relationship. This study therefore investigated the causal relationship between autoimmune diseases and breast cancer using the Mendelian randomization approach. Summary statistical data were obtained from genome-wide association studies to investigate the causal relationship between different autoimmune diseases including Graves' disease, Sjögren's syndrome, Crohn disease, systemic sclerosis, and psoriasis and breast cancer risk. The results revealed no strong evidence to support the causal relationship between Graves disease (odds ratio [OR] = 0.9958; 95% confidence interval [CI]: 0.9982-1.0035), Sjögren's syndrome (OR = 1.0018; 95% CI: 0.9950-1.0087), Crohn disease (OR = 2.1195, 95% CI: 0.1872-23.9978), systemic sclerosis (OR = 1.0024; 95% CI: 0.997-1.0079), and psoriasis (OR = 1.0016; 95% CI: 0.9913-1.0121) to breast cancer risk. A similar result was obtained in the reverse Mendelian randomization analysis. Our study provides evidence that autoimmune diseases might not have a causal relationship with breast cancer risk in the European population and concludes that shared genetic effects or environmental confounders.Copyright © 2023 the Author(s). Published by Wolters Kluwer Health, Inc.