We argue that growth (vs. fixed) mindsets are important for positive emotions and self-development because growth mindsets can foster adaptability, referring to the ability to adjust oneself in different circumstances. This study examines the role of mindsets in adaptability and whether adaptability, in turn, predicts learning emotions (anxiety and enjoyment), self-concept, and self-efficacy. The data were collected through self-report questionnaires from 211 (141 females and 70 males, M[subscript age] = 17.2 years, SD[subscript age] = 6.8) Iranian intermediate language learners. The path analysis results showed that fixed mindsets negatively predicted anxiety, enjoyment, self-concept, and self-efficacy through the mediation of adaptability, whereas growth mindsets positively predicted enjoyment, self-concept, and self-efficacy and negatively predicted anxiety through adaptability. The results held even after accounting for ideal L2 self and perceived competence. These findings highlight that growth mindset is an essential factor for developing positive learning emotions and self in foreign language classrooms.