The aim of this study is to understand the characteristics and meaning of the online platform-based adult learning experience by autobiographically describing the YouTube-based learning experience of an adult learner. The study results are briefly presented as follows. Firstly, as an adult learner, I converted the fragmented information on YouTube into systematic self-directed knowledge based on my subjective point of view and perspective. Secondly, as a researcher, I actively participated in YouTube-based online learning based on self-direction and mutuality. Thirdly, as an adult learner, I was able to overcome the limitations of the physical environment through the expansion of temporality and spatiality on YouTube. The results showed that the essence of the YouTube-based learning experience of an adult learner was that it is informal learning in which self-directed knowledge was subjectively constructed through self-awareness and reflection on the learning content, learning method, and learning environment mediated by YouTube. The results of this study will contribute to providing a theoretical and practical foundation for informal learning to promote the continuous growth and development of adult learners by identifying the characteristics and educational meaning of the YouTube-based online learning experience of an adult learner.