Sustainable energy is one of the biggest global challenges today. This paper discusses how we can promote adolescents' learning of sustainable energy with the help of an international massive open online course (MOOC). The aim of this case study is to understand: (1) What do the adolescents find relevant in the MOOC course about sustainable energy? and (2) What are the opportunities and challenges of the MOOC for the adolescents to learn sustainable energy? In our study, 80 voluntary adolescents around the world, who were at least 15 year old, took part in two surveys. The themes of our MOOC course were, e.g., sustainable growth, solar power, wind power, biofuel production and smart power generation. This 38 work-hour, free of charge, online course includes an introduction video, interviews of specialists, lecture videos, reading materials of the newest research and multiple choice questions on the topics. Research data was classified by using content analysis. The study indicates that adolescents feel that both the MOOC course and sustainable energy as a subject are relevant to them. Their decision to take part in an online course was mostly influenced by individual relevance and partly influenced by both societal and vocational relevance, according to the relevancy theory used. The MOOC was experienced to be relevant for the three following reasons: (1) good content (e.g., energy production) and implementation of the course; (2) the course makes it possible to study in a new way; and (3) the course is personally useful. The characteristics of the MOOC, such as being available anywhere and anytime, free access, and online learning, bringing out a flexible, new way of learning and thus promoting Education for Sustainable Development (ESD) in the context of sustainable energy at school level around the world. This MOOC provided the school students with choice-based learning and expanded their learning opportunities in understanding sustainable energy. In the designing of MOOCs for studying sustainable energy, it is important to take the following things into consideration: (1) the balance between theory and practical examples; (2) the support for interaction; and (3) other support (e.g., technical and learning strategies) for students. Communication with other learners and getting feedback from teachers and tutors remain the vital challenges for the developers of MOOCs in the future.