Abstarct: While teacher collaboration has been widely acknowledged as a valuable component of professional growth, its role in the professional identity development of English language teachers, which is the basis of effective teaching, has received limited scholarly attention, particularly within the context of Iranian English language institutes. Thus, understanding the role of teacher collaboration and its implications for professional identity development of teachers is essential, which results in teachers’ stronger sense of self-perception and eventually their success. This study aims to investigate the reported practices of English language teachers regarding the role and implications of teacher collaboration for their professional identity development and the forms of collaborative activities that play an important role in this process. This qualitative study draws on in-depth semi-structured interviews in three sessions with 8 participants who teach in the institutes and were selected baxsed on purposive sampling to uncover their experiences and perceptions. The thematic analysis of the findings revealed four major themes as the implications of collaboration for professional identity development, which were Pedagogical Skills Development through Collaboration, Transformation of Perspectives through Collaboration, Emotional Support through Collaboration, and Reflective Development through Collaboration. It was reported that through these concepts, collaboration contributes to the development of teachers’ professional identity by changing their beliefs, attitudes, views, and descxriptions of themselves and their role as teachers. Additionally, three collaborative forms, which were Group Discussions and Meetings, Peer Observation and Feedback, and Peer Coaching and Feedback were emerged from what teachers reported as a form of collaborative activities that have an important role in their professional identity development. These findings carry important implications for teachers, administrators, and policy-makers seeking to foster collaborative environments that support teachers’ professional growth and identity formation. Moreover, it raises awareness of teachers themselves, which can lead to their greater success in their teaching.