Background: Body mass index (BMI) is defined as a poor prognostic factor in patients with breast cancer (BC). However, there are controversial results regarding the various effects of BMI on BC. This paper aims to investigate the association between BMI at presentation and the histopathological features and molecular subtypes of the tumor in patients with BC. Methods: A total of 3767 BC patients were enrolled in this study. All patients’ BMI at the time of initial diagnosis and tumor demographics were recorded. BMI was stratified into 3 groups as normal-weighted (BMI < 25 kg/ m2), over-weighted (BMI = 25-29.9 kg/ m²) and obese (BMI ≥ 30 kg/ m²). Molecular subtypes were categorized into 4 groups as luminal, HER2/luminal, HER2 over-expressive and triple negative breast cancer (TNBC), according to the ER/PR and HER2 status. Distribution of molecular subtypes, tumor characteristics and overall survival (OS) analysis according to the BMI groups were evaluated Results: Obesity was significantly associated with older ages at presentation when compared to normal and overweight BMI in both premenopausal (P < 0.001) and postmenopausal period (P < 0.001). TNBC was significantly more frequent in premenopausal patients with BMI ≥ 30 kg/ m² compared to BMI < 30 kg / m² (P = 0.005). Additionally, premenopausal patients with BMI ≥ 30 kg/ m² had less common luminal subtype (P = 0.029) and more frequently presented with higher tumor stage (P = 0.012) and tumor grade (P = 0.004) in comparison to patients with BMI < 25 kg / m². On the other hand, premenopausal patients with BMI < 25 kg / m² had significantly more ER positive tumors (P < 0.001) compared to their counterparts with BMI ≥ 25 kg/ m². Premenopausal obese patients with TNBC (P = 0.001) and luminal subtype (P = 0.002) had significantly shorter OS duration compared to overweight counterparts. Conclusions: Our data indicated that BMI at presentation was associated with a decreased incidence for luminal and an increased incidence for TNBC subtype among premenopausal patients. However, this significance was not present in postmenopausal patients. Accordingly, a plausible etiological heterogeneity in BC might play a role among molecular subtypes in every life stages of women.