This study adopted the modified Delphi method to build key competence indicators for the course subject 'Environment' in the secondary school curriculum in South Korea. 15 Delphi panelists participated, and were provided a three-stage questionnaire survey. The findings classified eight secondary school 'environment' curriculum key competencies into three domains: intellect-oriented, personality-oriented, and relationship-oriented. The intellect-oriented domain includes 'critical-thinking ability,' 'creativity,' and 'problem-solving ability'; the personality-oriented domain includes 'autonomy,' 'ability to reflect,' and 'environmental sensitivity'; and the relationship-oriented domain includes 'communication ability' and 'ability to manage conflicts.' These environmental competencies were utilized to revise the current environmental education curriculum as part of the 2015 revision of the National Curriculum and set the key competencies for environment classes. The paper concludes with the significance of developments in theorizing and implementing environmental education curriculums in Korea and abroad.