Abstract Background School-based nutrition education programs have increasingly been used to teach children about healthy and sustainable nutrition, but there are less recognized programs available for secondary education. In this study we studied the effectiveness of the digital ‘Know what you eat’ program from the Netherlands Nutrition Centre among students (12-15y) on behavioral determinants and self-reported behaviors related to healthy, safe, and sustainable eating behavior. Methods Within a quasi-experimental research design, students in the first or second grade of 7 intervention schools and 11 control schools received a digital questionnaire during the pretest and posttest measurement. The questionnaire included questions on knowledge, self-efficacy, attitude, subjective norm, intention, and behavior related to healthy, safe, and sustainable eating behavior. Results The pre- and post-questionnaire was filled out by 364 students in the intervention and 247 in the control group. Linear multilevel regression analyses show a higher significant increase in self-efficacy, attitude, intention to drink water (all three p < 0.01), and a significant decrease in sugary drinks, snacks and meat consumption (all p < 0.05) in the intervention group than the control group. Additionally, students in both the intervention and control groups scored higher on knowledge during the post-test than the pre-test (both p < 0.05), but the intervention group was not significantly higher than the control group (p = 0.14). No significant effects were found for subjective norm, intention, and fruit, vegetable, and whole grain bread consumption. Conclusions The results of this study showed effects of the ‘Know what you eat’ program on self-efficacy and attitude towards healthy, sustainable, and safe eating behavior, intention to drink more water, and several healthy eating behaviors of secondary school students. Further research is needed to determine the sustainability of these results in the long term. Key ...