Active surveillance (AS) is increasingly used to monitor patients with low-risk prostate cancer; however, approximately 50% of AS patients experience disease reclassification requiring definitive treatment and little is known about patient characteristics that modify the risk of reclassification. Obesity may be one of the major contributing factors. The Prostate Cancer Active Lifestyle Study (PALS) is a clinical trial evaluating the impact of weight loss among overweight/obese (Body Mass Index (BMI) ≥ 25 kg/m) men with clinically localized prostate cancer on AS. Two hundred participants will be randomized to either the PALS intervention, a 6-month structured diet and exercise program adapted from the Diabetes Prevention Program followed by 6 months of maintenance, or control (general diet and physical activity guidelines delivered in a single session). The PALS intervention involves one-on-one instruction with a registered dietitian and exercise physiologist to achieve the study goal of loss of 7% of baseline weight. Participation is coordinated so that the 6-month time point coincides with the participants' standard-of-care AS prostate biopsy. Primary outcomes will evaluate the intervention effects on circulating and tissue markers of glucose and insulin regulation, health-related quality of life and pathologic upgrading on follow-up prostate biopsies. Additional analyses will determine whether changes in weight and glucose regulation can be sustained for 6 months after the end of instruction. Findings from this trial may have wide reaching implications for men diagnosed with clinically-localized prostate cancer by providing an active lifestyle-based approach to improve prostate cancer patient outcomes.