Introduction. Healthy lifestyle promotion programs are essential to prevent metabolic diseases such as obesity and diabetes. Adolescents could represent the ideal target population especially using serious web games. Objective. To test the improvement of knowledge about healthy nutrition in adolescents and to analyze participants’ enjoyment in playing the web game “Gustavo in Gnam’s Planet” in comparison with a leisure web game. Subjects and Methods. Sixty-five adolescents were engaged in three supervised group sessions. Measures about healthy food knowledge and games’ enjoyment were collected during the three stages of assessment. Results. After playing Gustavo in Gnam’s Planet, participants significantly improved their knowledge on a healthy diet, compared to the recreational web games; whereas the level of fun experienced while playing the recreational and the educational games was not significantly different. Conclusions. Gustavo in Gnam’s Planet is an important promising tool, with entertainment property, to promote a healthy lifestyle in Italian adolescents.