Obesity and the unsustainability of food systems are intertwined global challenges that demand coordinated solutions. Diets high in energy-dense, ultra-processed foods both elevate obesity risk and carry substantial environmental footprints; meanwhile, policies that prioritize volume and convenience entrench resource-intensive production and obesogenic food environments. Using a systems lens, we map feedbacks across supply chains, food environments, and social inequities that couple obesity and environmental degradation. We propose the Health in All Policies (HiAP) framework to align health and sustainability objectives and overcome sectoral silos. Cross-sector policy bundles—spanning agriculture and procurement, fiscal measures, urban planning and transport, marketing and school nutrition, and trade—can deliver “double-duty” benefits while minimizing unintended consequences. We outline implementation principles (equity, accountability, and co-benefit indicators such as diet quality, affordability, greenhouse-gas emissions, and land use) to guide evaluation. This perspective reframes obesity prevention and food system sustainability as mutually reinforcing goals, offering an actionable, HiAP-guided roadmap with practical tools and context-adaptable policy bundles to disrupt their syndemic interplay through integrated, multi-sectoral action.