Background: Excess weight is currently recognized as a risk factor for several cancer types, including breast cancer. The primary goal of this study was to evaluate the impact of overweight and obesity in newly diagnosed breast cancer patients at the time of presentation. Methods: A retrospective analysis of breast cancer from a prospective database of all newly diagnosed non-metastatic breast cancer patients seen at King Faisal Specialist Hospital and Research Center between 2002 and 2014 was performed. The clinical stages were divided into early stage breast cancer and locally advanced breast cancer. The body mass index (BMI) groups were underweight, normal, overweight, and obese based on the World Health Organization classifications of BMI. The patient characteristics are presented as medians with interquartile ranges (IQRs) and frequencies for continuous and categorical variables, respectively. The association between BMI groups and clinical stage at presentation was evaluated using the logistic regression model. Survival probabilities were calculated using the Kaplan-Meier estimator. Results: In total, 2212 patients were eligible for the study. The median age at diagnosis was 45 (IQR = 39-52) years; 62% patients were pre-menopausal, and 31% were post-menopausal. The median BMI was 30 (IQR = 26-34) kg/m². In this population, 53% patients were obese; 31%, overweight; and 14.7%, in the normal range at diagnosis. Regression analysis revealed a significant association between clinical stage and BMI at the time of presentation (p = 0.006). Obese patients showed a 40% higher chance of having locally advanced presentation than the normal BMI group (OR = 1.41, 95% confidence interval = 1.06-1.86, p = 0.02). However, overweight had no significant association with clinical stage (OR = 1.03, 95% confidence interval = 0.76-1.8). The median follow-up duration was 39 (IQR = 22-66.6) months. Overall survival showed no significant association with different BMI groups and breast cancer subtypes. Conclusions: The prevalence of overweight and obesity was found to be high (85%) in newly diagnosed breast cancer patients in Saudi Arabia. Obesity is associated with a more advanced clinical stage at the time of diagnosis of breast cancer and may be a contributing factor for more locally advanced presentations in the region.