As the trend for English as a medium of instruction (EMI) increases, teacher self-efficacy to teach the contents of their teaching materials in an EMI context has become one of the major challenges. This study investigated the self-efficacy of Indonesian non-ELT pre-service teachers to teach in an EMI context. Two Indonesian non-ELT pre-service teachers, in charge of teaching biology and chemistry in English, in a secondary school in Thailand were interviewed to gain an indepth information about their experiences. The data from their teaching reflections and interviews were analyzed using thematic analysis. The study shows that both participants perceived that their self-efficacy to teach in EMI context was quite high despite various obstacles during their teaching practicum. Furthermore, four sources of efficacy information appeared to influence the Indonesian non-ELT pre-service teachers' self-efficacy to teach in the Thailand EMI context. Mastery experience (the personal experience of success) was the most influential source of efficacy information, followed by the other instances of self-efficacy sources including social persuasion, emotional state, and vicarious experience. The implication derived from this study is that the teacher training institutions in Indonesia need to facilitate the pre-service teachers to help them perceive their self-efficacy to teach in EMI context.