<p>Obesity’s impact on breast cancer prognosis may depend on molecular subtype, yet there is limited research on premenopausal women with HR+HER2- breast cancer. In this study, we included 5,094 premenopausal women with HR+HER2- early-stage breast cancer who underwent radical surgery at 42 breast centers across China between 2016 and 2021. Patients were categorized into four BMI groups: underweight (UW), normal weight (NW), overweight (OW), and obese (OB). Using propensity score matching (PSM) to adjust for confounding factors, we compared disease-free survival (DFS) across BMI groups with Kaplan–Meier curves and performed multivariate Cox regression analysis to evaluate the effect of obesity on prognosis. Our results showed that obese patients had significantly worse DFS compared to those who were UW, NW, or OW (<i>p</i> = 0.013). After adjusting for clinicopathologic factors and treatment, obesity remained an independent prognostic factor for DFS in premenopausal HR+/HER2- patients (<i>p</i> = 0.043). After 1:1 PSM, OW/OB patients receiving selective estrogen receptor modulators (SERMs) alone had significantly worse DFS compared to UW/NW patients (<i>p</i> = 0.022), whereas OW/OB patients treated with SERMs or aromatase inhibitors (AIs) in combination with ovarian function suppression (OFS) showed no significant difference in DFS from UW/NW patients. In addition, Restricted Cubic Splines (RCS) analysis demonstrated that the risk of recurrence increases with BMI in OW/OB patients. Collectively, these findings indicate that obesity may act as an independent adverse prognostic factor for DFS in premenopausal women with HR+/HER2− breast cancer. Importantly, our results also suggest that OFS may attenuate the detrimental effect of obesity in patients treated with SERMs.</p>