Quite a few estrogen receptor (ER)-positive breast cancer patients receiving endocrine therapy are at risk of disease recurrence and death. ER-related genes are involved in the progression and chemoresistance of breast cancer. In this study, we identified an ER-related gene signature that can predict the prognosis of ER-positive breast cancer patient receiving endocrine therapy. We collected RNA expression profiling from Gene Expression Omnibus database. An ER-related signature was developed to separate patients into high-risk and low-risk groups. Patients in the lowrisk group had significantly better survival than those in the high-risk group. ROC analysis indicated that this signature exhibited good diagnostic efficiency for the 1-, 3-and 5-year disease-relapse events. Moreover, multivariate Cox regression analysis demonstrated that the ER-related signature was an independent risk factor when adjusting for several clinical signatures. The prognostic value of this signature was validated in the validation sets. In addition, a nomogram was built and the calibration plots analysis indicated the good performance of this nomogram. In conclusion, combining with ER status, our results demonstrated that the ER-related prognostic signature is a promising method for predicting the prognosis of ER-positive breast cancer patients receiving endocrine therapy.