This longitudinal study examined how an English for Specific Purposes (ESP) teacher transformed her teacher identity over time. Hinging on the close relationship between teaching and identity construction, this study was grounded on the notion that teaching a language is becoming a person who teaches it (Benson, 2017). Data were collected through six face-to-face interviews and six electronic interviews with the teacher during a four-year period. Findings demonstrate that becoming an ESP teacher is a complex process of constructing and negotiating identities. The teacher started ESP teaching as a scared general English teacher, and then negotiated her identities as a struggling but hardworking collaborator and learner. Later, she positioned herself as a novice ESP teacher, and then finally constructed her identity as a competent, helpful ESP teacher. Significant factors affecting identity transformation include needs analysis, enhanced subject matter knowledge, capacity to develop teaching materials, and teacher agency. Based on these findings, this study suggests implications for ESP teacher education programs.