Background: By definition, messages in tailored approaches are built to reach a given person, based on her/his specific characteristics and needs. Tailored dietary behavior change interventions have a small but significant effect on dietary behavior change. The majority of these interventions target a few food groups or nutrients, without evaluating the overall diet. Diet optimization is a powerful mathematical method to translate nutrient recommendations into individual-specific food choices. This method is increasingly used in nutrition research, in the fields of public health and diet sustainability. Objective: The aim was to combine tailored approaches and diet optimization in a web application of tailored nutrition counselling. Method and results: The web application, called MS-Diet, was developed based on behavior changes techniques, such as: self-monitoring, self-regulatory, tailored feedback and engaging communication techniques. In a first feature, based on user’s data collected online (including answers to a food frequency questionnaire), the user can obtain a picture of the nutritional quality of her/his diet, the diet cost and the level of physical activity. In a second feature, MS-Diet suggests to the user a list of tailored dietary advices to get a healthier diet (i.e., a nutritionally adequate diet), adapted to her/his specific needs and food preferences. With the application, the user is actor in her/his own dietary changes: she/he specifies her/his food preferences and; chooses dietary suggestions that she/he considers achievable. Conclusions: This prototype could be a future online health promotion tool which could help individuals to improve their diet or serve as a decision-support tool for health professionals. The evaluation of the tool (e.g. whether the use of the tool results in changes of dietary habits) is warranted before use on health promotion.