Sustainable diets are an increasingly debated policy concept to address many of the environmental, social, and economic issues in the food system. The role of ultraprocessed foods in sustainable diets has received less attention than meat, dairy, and eggs but is deserving of examination given the high environmental impacts and negative health outcomes resulting from consumption of these foods. Big Food companies that make ultraprocessed foods have focused their attention on sustainable sourcing as a significant sustainability strategy. This article argues that sustainable sourcing as a central strategy for Big Food firms has implications for the achievement of sustainable diets. First, sustainable sourcing lends legitimacy to specific discourses of sustainability that align with a growth imperative. Second, it perpetuates weak and fragmented governance, which can enhance the legitimacy of Big Food when participating in coordination efforts. These dynamics of sustainable sourcing are important for consideration given the legitimacy claims of these companies, which situate them as a key part of the solution in working toward food security and sustainability.