<p>Breast cancer has now become the most common malignant tumor and the leading cause of cancer deaths in women worldwide, and its pathogenesis has not yet been elucidated. Obesity is a risk factor for multiple cancers, and with the improvement of living standards and changes in dietary structure, obesity has become a major public health challenge in Asia, North America, Europe, and even worldwide. The study of obesity and breast cancer risk is a popular topic in epidemiological research. However, due to variations in obesity assessment criteria, conclusions regarding breast cancer risk in premenopausal versus postmenopausal women have been inconsistent. This review systematically reviews diverse diagnostic criteria for obesity from domestic and international studies, and thoroughly analyzes the differential associations between obesity and breast cancer risk in premenopausal and postmenopausal women, along with their potential molecular biological mechanisms. This paper further summarizes the clinical characteristics of breast cancer patients with obesity, including delayed diagnosis, poor pathological features, and unfavorable prognosis. At the mechanistic level, we integrate multidimensional evidence spanning obesity-related genes (e.g., fat mass and obesity associated genes (<i>FTO</i>)), adipokines (e.g., leptin (LEP), resistin, visfatin, adiponectin (ADPN)), estrogen metabolism, chronic inflammation, and tumor microenvironment remodeling (including cancer-associated adipocytes, extracellular matrix, and microbiota). Finally, this paper envisions the broad prospects of bariatric metabolic surgery in future adjuvant breast cancer treatment. By comprehensively employing lifestyle interventions, drug therapies, and bariatric metabolic surgery, we can provide patients with more comprehensive and personalized treatment plans to achieve better therapeutic outcomes and prognosis.</p>