This study investigated the effects of outdoor education on a 10th grade biology curriculum in terms of the knowledge, attitudes, and retention of students compared to indoor teaching. The quasi-experimental design included three groups: experiment (OE; N = 43), control-II (indoor teaching; N = 35), and control-I (no application; N = 35). A multiple-choice knowledge test and biology attitude scale assessed the changes in knowledge and attitudes. The experiment and control-II groups took pre-, post-, and retention-tests, whereas the control-I group took pre- and retention-tests. Overall, the experiment group's knowledge and attitudes were more positively affected than those of control-II. Outdoor Education had a more positive effect on retention and attitudes than did indoor teaching; it took less time and was more economical and temporarily more efficient. Adding Outdoor Education to secondary education biology curricula would increase students' knowledge and positive attitudes. Cooperation between policy makers and universities is necessary for its effective application.