Therapeutic education in diabetes helps patients take responsibility for self-control of their disease, and providing technological support systems facilitates this education. In this paper, we present an augmented reality game to support therapeutic education for patients with diabetes. Our game helps children (aged 5-14 years) to learn carbohydrate (carb) content of different foods. The game shows virtual foods on a real dish. The number of carb choices corresponding to the visualized food is also shown (1 carb choice = 10 grams of carbs). A study to determine the effectiveness of the game in terms of learning and perceived satisfaction and usability was carried out. A total of seventy children with diabetes participated in the study. From the results, we observed that the initial knowledge about carb choices of the children who participated in the study was low (a mean of 2 on a scale from 0 to 9). This indicates that therapeutic education for patients with diabetes is needed. When the results for the pre-knowledge questionnaire and the post-knowledge questionnaire were compared, it was shown that the children learned about carb choices by playing our game. We used two post-knowledge questionnaires (one post-knowledge questionnaire that contained the same foods as the pre-knowledge questionnaire and a second post-knowledge questionnaire that contained foods that were different from the ones on the pre-knowledge questionnaire). There were no statistically significant differences between these two different post-knowledge questionnaires. Moreover, the knowledge acquired was independent of gender and age. We also evaluated usability and perceived satisfaction. The children were satisfied with the game and considered that the game offers a high degree of usability. This game could be a valuable therapeutic education tool for patients with diabetes.