Pakistan has recently made STEM part of its mainstream education, as evident by the National Curriculum of Pakistan for Science (2022-23). Hence, the inclusion of STEM in the science curriculum places sole responsibility on the science teachers for implementing STEM reform at the classroom level. This makes it inevitable to explore the transformation of a science teacher to a STEM teacher. Seeing the novelty of the idea of STEM teacher identity in the context, the study was purposefully delimited to a single participant teacher using a single-case holistic design to explore the participant teacher’s unique STEM experiences that shaped her STEM teacher identity. Data collected from interviews, self-portrait as a teacher in the science classroom, critical incidents, artifacts and document analysis helped in gaining deep insights into the complex process of identity development. Participant teacher’s journey depicted continuous learning leading to identity transformation due to the interplay of personal, professional and social experiences in the teaching and learning context of the participant. Participant teacher’s beliefs and experiences as a learner of science, her career-changing experiences of becoming a science teacher, her reform-minded beliefs and actions considering contextual negligence of science teaching in the classroom, especially her participation in STEM teacher preparation programme and workshops visibly contributed to the evolution of her STEM teacher identity. Particularly, the process of evolution of identity was reflected through her diverse STEM specific experiences as a STEM learner during professional development, as a curriculum developer, as a teacher coach, and as a researcher. Yet, it does not conclude that the participant’s identity transformed completely, fulfilling entirely the expectations of STEM education. The study also provides recommendations that have implications for all educational stakeholders to make appropriate arrangements for integration of STEM and especially calls for ongoing STEM-focused professional development of teachers