Recently the concept of the collaborative teacher learning has emerged as a popular and powerful strategy for teachers' professional development. The strength of collaborative teacher learning is that it encourages workplace learning and develops a sense of continuous professional growth among teachers. More importantly, the role of the headteacher in bringing about any kind of change in the school premises is found to be very crucial. Due to their position, authority and influence within the school, they can bring improvement in their own practices and also encourage and involve other colleagues to do the same. The purpose of this study was to explore the role of a headteacher in fostering collaborative learning among teachers in a government boys' secondary school in Karachi, Pakistan. For this study, the case study design was employed under the qualitative research paradigm. Research participants included the headteacher of the school as the main research participant and four teachers from different grade levels, with different experiences. The purpose of selecting such teachers was to generate data from various perspectives. Data were collected through interviews, observations, document analysis and informal talks. All the recorded data were first transcribed, analyzed by coding and categorized during the fieldwork. After the fieldwork, summative data analysis was employed. The study reveals that the headteacher plays a very significant role in fostering collaborative learning among teachers. He has created such an environment that provides teachers opportunities to learn from each other by involving them in activities that require working together as a team, guiding and supporting each other. The headteacher continuously encouraged teachers to build confidence and helped them in utilizing their own expertise and capabilities through sharing and discussions for better teaching and learning practices. This shows that the concept of collaborative teacher learning can be used as a very successful strategy for teachers' continuous professional development in the government schools in the context of Pakistan, where there are few opportunities for teachers to join professional development institutes.