Purpose <p>To evaluate the effect of body mass index (BMI) on oncologic outcomes in patients with breast cancer stratified by menopausal status and histological subtype. Although studies have focused on the relationship between obesity and breast cancer risk, the association between BMI and breast cancer recurrence after surgery remains controversial.</p> Methods <p>This retrospective study included patients who underwent curative surgery for breast cancer between June 2003 and November 2017. Normal weight and overweight groups were defined based on the World Health Organization classification. The primary outcome was recurrence-free survival, evaluated at 1, 5, and 10&#xa0;years after curative surgery. Patients were stratified by BMI category, histological subtype, and menopausal status. The main measures included tumor characteristics, recurrence events, and survival outcomes across groups.</p> Results <p>Among 4506 patients included in the analysis, 3384 (75.1%) had luminal-type breast cancer. The overweight group (n = 1259) was associated with older age (normal weight (NW): 50.2 ±10.9 vs. overweight (OW): 56.5 ± 1.9, <i>P</i> &lt; 0.001) and higher T stage (≥ T2: NW: 1226 (37.7%) vs. OW: 577 (45.8%), <i>P</i> &lt; 0.001). In patients with luminal-type breast cancer, 10-year recurrence-free survival was significantly worse in the overweight group (NW 89.3% vs. OW 85.7%, <i>P</i> = 0.018). Subgroup analysis showed that premenopausal patients with luminal-type breast cancer who were overweight had an increased risk of recurrence (<i>P</i> = 0.003).</p> Conclusions <p>Obesity is a significant, potentially modifiable risk factor for recurrence in premenopausal females with luminal-type breast cancer.</p>