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Melanoma remains a major global health burden, though immunotherapy and targeted therapy have markedly improved survival. Obesity has paradoxically been associated with favorable outcomes in melanoma, yet body mass index (BMI) alone fails to capture its influence on treatment response. To address this gap, we conducted a multi-site cohort study within the Canadian Melanoma Research Network, including patients with advanced melanoma treated with immunotherapy or targeted therapy. Body composition was quantified using computerized tomography (CT) imaging to assess visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), skeletal muscle (SM) mass and intermuscular adipose tissue (IMAT), and associations with progression-free survival (PFS) and overall survival (OS) were evaluated. No overall association was seen for BMI, SAT, VAT, IMAT or SM with PFS or OS. In the targeted therapy subset, higher BMI, SAT, VAT and SM were associated with better OS (hazard ratios 0.56 to 0.65), while no effect was seen in the immunotherapy group. IMAT emerged as a novel prognostic marker, with elevated levels associated with lower OS in males and better OS in females. Our findings show that CT-based body composition is not associated with survival outcomes in patients with advanced melanoma receiving immunotherapy.