OBJECTIVE: Type 2 diabetes mellitus is a chronic metabolic disorder characterized by insulin resistance and β-cell dysfunction, often coexisting with obesity. Obesity increases adipose tissue inflammation and oxidative stress, which may impair mitochondrial function and muscle metabolism. The aim of this study was to investigate the effects of obesity severity on oxidative stress biomarkers, exercise-related biomarkers, physical activity level, and sleep quality in individuals with type 2 diabetes mellitus. METHODS: This prospective observational study included 80 individuals aged 20–80 diagnosed with type 2 diabetes mellitus. Participants’ body mass index, physical activity assessed by the International Physical Activity Questionnaire-Short Form, and sleep quality assessed by the Pittsburgh Sleep Quality Index were recorded. Serum samples were analyzed for oxidative stress biomarkers (total antioxidant status, total oxidant status, malondialdehyde, and superoxide dismutase) and exercise-related biomarkers (irisin, Fibronectin Type III Domain-Containing Protein 5, Adenosine Monophosphate-Activated Protein Kinase, and leptin) using enzyme-linked immunosorbent assay methods. RESULTS: There were significant differences in irisin, Fibronectin Type III Domain-Containing Protein 5, Adenosine Monophosphate-Activated Protein Kinase, leptin, malondialdehyde, superoxide dismutase, total antioxidant status, and total oxidant status values according to body mass index levels (p<0.001). Overweight participants had higher irisin, Adenosine Monophosphate-Activated Protein Kinase, and Fibronectin Type III Domain-Containing Protein 5 levels, while leptin, malondialdehyde, and total oxidant status levels increased with obesity severity. Physical activity levels decreased as body mass index increased (p=0.016). No significant differences were observed in self-reported sleep quality parameters assessed by the Pittsburgh Sleep Quality Index across body mass index groups (p>0.05). CONCLUSION: Increasing obesity severity in individuals with type 2 diabetes mellitus is associated with elevated oxidative stress and decreased antioxidant defense, along with lower levels of exercise-related metabolic biomarkers. These findings highlight the importance of promoting regular physical activity and interventions targeting the irisin/Adenosine Monophosphate-Activated Protein Kinase pathway to reduce oxidative stress and improve metabolic health in obese diabetic individuals.