Over the last decades, the abandonment of traditional dietary patterns, such as the Mediterranean diet, represents an important threat for human health and environmental safeguard. The DELICIOUS project aims to promote healthy lifestyles among children and adolescents by implementing activities and tools to increase the adherence to the Mediterranean Diet with an attention to the environmental impacts of the diet. This study protocol describes the DELICIOUS project as a single-arm, uncontrolled behavioural intervention providing formal and non-formal education activities, development of new snacks and recipe reformulation, web/mobile app development, and physical activities to school children and adolescents in five European countries. The project aims to increase awareness of the nutritional benefits and the sustainability aspects of the Mediterranean Diet and to promote consumers’ empowerment through an online platform for sustainable and healthy meal planning in the school canteen.