Ultra-processed foods (UPF) have been identified as a key driver of obesity, with observational and clinical studies showing positive associations between UPF intake and weight gain, overweight, and obesity. UPF is defined as industrial formulations with five or more ingredients, often containing additives and processed culinary ingredients. The NOVA classification system categorizes foods based on processing extent and purpose, with UPF being the most processed group. UPF is associated with higher energy density, lower nutrient density, and increased consumption of free sugars, fats, and saturated fats, while reducing intake of fibre, protein, and micronutrients. These factors contribute to energy overconsumption and weight gain. Mechanisms linking UPF to obesity include higher energy density, altered taste and satiety, hyperpalatability, and displacement of healthier food choices. Experimental studies show that UPF diets lead to greater energy intake and weight gain compared to minimally processed foods (MPF). However, the effects of UPF may not be solely due to processing but could also be influenced by specific components like sugar-sweetened beverages. While some UPF products may be considered 'healthy' due to lower energy density, the overall impact of UPF on obesity remains significant. The evidence suggests that UPF consumption is a major contributor to the obesity epidemic, and reducing UPF intake is crucial for public health. However, current dietary guidelines may not fully address the unique characteristics of UPF. Future research should focus on methodologically rigorous studies to clarify the role of UPF in obesity and inform policy. The NOVA classification system is a novel approach that may help in developing targeted interventions to reduce UPF consumption and promote healthier dietary patterns. Despite the growing body of evidence, further research is needed to understand the complex interactions between UPF, diet, and obesity.Ultra-processed foods (UPF) have been identified as a key driver of obesity, with observational and clinical studies showing positive associations between UPF intake and weight gain, overweight, and obesity. UPF is defined as industrial formulations with five or more ingredients, often containing additives and processed culinary ingredients. The NOVA classification system categorizes foods based on processing extent and purpose, with UPF being the most processed group. UPF is associated with higher energy density, lower nutrient density, and increased consumption of free sugars, fats, and saturated fats, while reducing intake of fibre, protein, and micronutrients. These factors contribute to energy overconsumption and weight gain. Mechanisms linking UPF to obesity include higher energy density, altered taste and satiety, hyperpalatability, and displacement of healthier food choices. Experimental studies show that UPF diets lead to greater energy intake and weight gain compared to minimally processed foods (MPF). However, the effects of UPF may not be solely due to processing but could also be influenced by specific components like sugar-sweetened beverages. While some UPF products may be considered 'healthy' due to lower energy density, the overall impact of UPF on obesity remains significant. The evidence suggests that UPF consumption is a major contributor to the obesity epidemic, and reducing UPF intake is crucial for public health. However, current dietary guidelines may not fully address the unique characteristics of UPF. Future research should focus on methodologically rigorous studies to clarify the role of UPF in obesity and inform policy. The NOVA classification system is a novel approach that may help in developing targeted interventions to reduce UPF consumption and promote healthier dietary patterns. Despite the growing body of evidence, further research is needed to understand the complex interactions between UPF, diet, and obesity.