Background: Hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-positive (HER2+) breast cancer exhibits considerable heterogeneity, and it is of great interest whether patients with premenopausal HR+/HER2+ breast cancer treated with trastuzumab can benefit from ovarian function suppression (OFS) therapy similarly to HR+/HER2- breast cancer. Here, we conducted a real-world study in this population to identify both who would derive substantial benefits from the addition of OFS and clinicopathological factors with potential prognostic value.Methods: Multicenter data from 253 premenopausal patients with HR+/HER2+ early-stage breast cancer who received trastuzumab from October 2009 to October 2018 were retrospectively included. The Kaplan-Meier method was used for survival analysis, while the log-rank test was used to compare the survival rates. Univariate and multifactor Cox regression analyses were performed to analyze the independent risk factors affecting invasive disease-free survival (IDFS).Results: After a median follow-up of 98.50 months, compared with tamoxifen/toremifene alone, tamoxifen/toremifene/aromatase inhibitors plus OFS demonstrated significant benefits in the overall study population (HR = 0.289, 95% CI: 0.100-0.835, p = 0.022, 8-year IDFS rate: 90.78% vs. 95.54%), especially in the lymph node-positive subgroup and age <= 40 years subgroup. Age <= 40 years, histological grade >2, lymph node involvement, PR <= 50%, and tamoxifen alone were independent prognostic factors.Conclusions: For premenopausal HR+ breast cancer patients, HER2 positivity alone is an indication for the addition of OFS in adjuvant endocrine therapy. Age, histological grade, lymph node status, the expression of PR, and OFS treatment were independent prognostic factors in this population.