This review critically appraises the evidence linking ultra-processed food (UPF) intake to obesity. Observational studies consistently show positive associations between UPF consumption and weight gain, overweight, and obesity, particularly in adults. High-quality clinical data further support these findings. Mechanisms proposed to explain the link include increased energy intake, altered texture, taste, and satiety, as well as environmental and behavioral factors. However, the mechanisms are not yet fully understood. The NOVA classification system, which categorizes foods based on processing extent and purpose, has been useful for identifying novel exposure-outcome associations. Despite methodological challenges, the evidence suggests that UPF is a significant driver of obesity. The review calls for collective action to change the obesogenic environment, including policy changes and the development of accessible, sustainable alternatives to UPF.This review critically appraises the evidence linking ultra-processed food (UPF) intake to obesity. Observational studies consistently show positive associations between UPF consumption and weight gain, overweight, and obesity, particularly in adults. High-quality clinical data further support these findings. Mechanisms proposed to explain the link include increased energy intake, altered texture, taste, and satiety, as well as environmental and behavioral factors. However, the mechanisms are not yet fully understood. The NOVA classification system, which categorizes foods based on processing extent and purpose, has been useful for identifying novel exposure-outcome associations. Despite methodological challenges, the evidence suggests that UPF is a significant driver of obesity. The review calls for collective action to change the obesogenic environment, including policy changes and the development of accessible, sustainable alternatives to UPF.