Objective To explore the causal relationship between breast cancer and rotator cuff injury using bidirectional two-sample Mendelian randomization. Methods Instrumental variables for breast cancer and rotator cuff injury were extracted from published genome-wide association study data. The positive study used breast cancer as the exposure and rotator cuff injury as the outcome, with single nucleotide polymorphisms (SNPs) closely associated with both breast cancer and rotator cuff injury as genetic instrumental variables. The reverse study used rotator cuff injury as the exposure and breast cancer as the outcome, with SNPs closely associated with both breast cancer and rotator cuff injury as genetic instrumental variables. Bidirectional MR analysis was conducted using five models: inverse variance weighted (IVW), simple model, weighted median, weighted model, and MR-Egger to assess the causal relationship between breast cancer and rotator cuff injury. Cochran Q test was used to detect heterogeneity, MR-Egger to detect horizontal pleiotropy, and leave-one-out method for sensitivity analysis to ensure the robustness of the results. Results A total of 51 SNPs closely associated with breast cancer were included in the forward study. The results indicated a positive causal association between breast cancer and an increased risk of rotator cuff injury [IVW: odds ratio=1.08, 95% confidence interval (1.02, 1.12), P=0.014], with no evidence of heterogeneity in the causal relationship between breast cancer and rotator cuff injury (P>0.05). Horizontal pleiotropy test results showed no horizontal pleiotropy in the SNPs (P>0.05). Leave-one-out test results did not detect any SNP with a large impact on the results. In the reverse study, a total of 3 SNPs related to rotator cuff injury were included as instrumental variables. There was no strong evidence that rotator cuff injury had a causal effect on breast cancer incidence [IVW: odds ratio=0.95, 95% confidence interval (0.86, 1.05), P=0.334]. Conclusions There is a potential causal association between breast cancer and rotator cuff injury. Therefore, it is suggested to increase the screening for rotator cuff injury in breast cancer patients.