Despite the recent growth of attention to language teachers' emotions, knowledge about how emotional tensions contribute to student teachers' identity construction is still under-developed. In response to this gap, the present study explored the role of emotional tensions in seven Iranian English language student teachers' identity construction over a semester-long period. Theoretically grounded in activity theory, the study was conducted through collecting data from pre-course interviews, during-course reflective journals, and post-course interviews. Data analysis revealed three major themes: (1) emotional tensions as a source of marginalisation, (2) emotional tensions as a motivator of professional investment, and (3) emotional tensions as a fund of identity. We found that beyond positive and negative contributions for student teachers' identity construction, emotional tensions could play a mediatory role in identity development when they interact with system-related discourses and participants. Based on the findings, we provide implications for teachers and teacher educators to turn emotional tensions into a positive contributor to interpersonal and organisational development.