This study was an inquiry to investigate English for Specific Purposes (ESP) teachers' transitional experiences in the third space. To that end, 17 ESP teachers wrote autobiographical narratives focusing on their critical challenges in the third space and their approaches to dealing with them. The narratives were analyzed using Creswell's (2007) procedures to analyze the narratives, including managing the data, reading and memoing, describing the memos, classifying, and interpreting. The findings showed that ESP teachers' main challenges in the third space were negotiation tensions, identity struggles, the inefficiency of their previous experiences, and institutional barriers to change. Furthermore, ESP teachers' approaches to evolving their professional identity in the third space encompassed addressing reflective teaching and criticality, seeking external support, and creating a third space community of practice. It can be concluded from the findings of the current study that the third space can be a double-edged sword for ESP teachers since, on the one hand, some tensions in the third space create some problems for their professional identity development. On the other hand, ESP teachers need to take some approaches to address the third space tensions, helping them evolve their professional identity.