Background: Higher body mass index (BMI) has been associated with an increased risk of developing hormone receptor (HR)-positive breast cancer (BC) in postmenopausal women, but its relationship with prognosis in patients with HR-positive BC remains unclear. In particular, there is limited data on BMI and prognosis in Asian patients with BC, whose BMI distribution differs from that of Westerners. Methods: We retrospectively reviewed data from Japanese women who underwent curative surgery for clinical stage I-III HR-positive BC in our institution between January 2007 and December 2017. The patients were classified into five groups based on BMI: underweight (UW), BMI < 18.5 kg/m²; normal weight (NW), 18.5 to 24.9 kg/m²; obese 1 degree (OB1), 25.0 to 29.9 kg/m²; obese 2 degree (OB2), 30.0 to 34.9 kg/m²; obese 3 degree (OB3), ≥ 35 kg/m². We compared the clinicopathological features between patients with OB1-3 and those with UW and NW. The prognostic analysis of breast cancer-specific survival (BCSS) and overall survival (OS) was performed using the log-rank test and Cox proportional hazards model between the two groups. Subgroup analysis of BCSS and OS was performed in premenopausal and postmenopausal patients. Results: Overall, 6,421 patients were included in the study. The median follow-up duration was 5.9 years (range, 0.2-12.9). The number of patients with UW, NW, OB1, OB2, or OB3 was 599 (9%), 4477 (70%), 1085 (17%), 224 (3%), and 36 (1%), respectively. Of these, 3380 (53%) were premenopausal and 2968 (46%) were postmenopausal. Patients with OB1-3 had a significantly higher stage of BC, a higher number of lymph node metastases, and were associated with higher Ki67 index and nuclear grade compared to those with UW and NW. A total of 521 all-cause and 307 breast cancer-specific deaths as well as 824 recurrences were observed. Both BCSS and OS in patients with OB1-3 were significantly worse than those in the UW and NW patients (hazard ratio [HR], 1.66; 95% confidence interval [CI], 1.14-2.42; HR, 1.64; 95%CI, 1.25-2.18, respectively) by univariate analysis. In addition, the Kaplan-Meier curve for each BMI group showed that the prognosis of BCSS and OS was gradually worse in the order of NW, OB1, OB2, and OB3. Subgroup analysis revealed that BCSS and OS were significantly worse in patients with OB1-3 than in those with UW and NW for premenopausal patients (HR, 2.37; 95%CI, 1.40-4.02; HR, 1.91; 95%CI, 1.17-3.11, respectively), but not for postmenopausal patients (HR, 1.26; 95%CI, 0.74-2.16; HR, 1.29; 95%CI, 0.92-1.82, respectively). Multivariate analysis showed that tumor stage and nuclear grade were independent prognostic factors, but OB1-3 was not. Conclusions: Obesity was associated with a worse prognosis as it correlated with advanced stage and high nuclear grade in Japanese patients with HR-positive BC, especially in premenopausal patients.