Abstract Introduction Obesity and breast cancer are two major pathologies closely associated with increasing incidence and mortality rates, especially amongst women. The association between both diseases have been thoroughly discussed but much is still to uncover. Aim The aim of this study is to analyse tumour characteristics and clinical outcomes of overweight and obese women to disclosure potential associations and better understand the impact of obesity in breast cancer. Materials and methods Clinicopathological information of 2246 women were extracted from the institutional database of comprehensive cancer centre in Portugal diagnosed between 2012 and 2016. Women were stratified according to body mass index as normal, overweight, and obese. Patients’ demographic information and tumour features (age, family history, topographic localization, laterality, histological type, and receptor status) were taken as independent variables and overall survival, tumour stage, differentiation grade and bilaterality were considered clinical outcomes. Results The main results reveal that overweight and obesity are predominantly associated with worse outcomes in breast cancer patients. Obese patients present larger (p-value: 0.002; OR 1.422; 95% CI 1.134–1.783) and more poorly differentiated tumours (p-value: 0.002; OR 1.480; 95% CI 1.154–1.898) and tend to have lower overall survival although without statistical significance (p-value: 0.117; OR 1.309; 95% CI 0.934–1.833). Overweighted women are more likely to have bilateral breast cancer (p-value: 0.017; OR 3.076; 95% CI 1.225–7.722) than obese women. The results also reveal that overweight women present less distant metastasis (p-value: 0.024; OR 0.525; 95%CI 0.299–0.920). Topographic localization and laterality did not achieve statistical significance.