This study developed the curriculum for ‘Our Ocean Action’, a transdisciplinary ocean education elective subject for School Autonomous Hours in middle schools, and analyzed its design principles, curriculum framework, and revision through the approval review process. Design and development research was employed. The development procedure was adapted into four stages: preparation, planning, development, and review/approval. Data comprised national and local curriculum documents, literature on ocean literacy and transdisciplinary education, a nationwide analysis of 359 approved subjects, draft and final curriculum documents, and ten review comments with revision records. Transdisciplinary design was examined using five criteria: real-life complex phenomena, reorganization of disciplinary knowledge, integration of knowledge-skills-values, connection of scientific-social-technological-policy perspectives, and extension to action. The curriculum organized ocean-human interaction as a macroscopic phenomenon and comprised three content areas, thirteen achievement standards, and a 34-period elective course for all middle school grades with pass/fail reporting. Representative learning phenomena, activities, and assessment evidence were specified at the curriculum level. Review feedback strengthened relevance to students’ lives, curricular progression, action-oriented learning, and assessment congruence. The curriculum received final approval, while classroom implementation and learning outcomes remain to be examined.