This study explored how the Self-Determination Theory (SDT) framework shaped physics teachers’ professional identities. Through a qualitative case study design, the researcher analyzed the teaching practices, interactions, and pedagogical preferences of two experienced physics teachers. The data sources included classroom observations, semi-structured interviews, informal conversations, and teaching materials. The male teacher adopted a traditional, authority-based approach, while the female teacher employed a student-centered, autonomy-supportive, and relationship-based teaching approach. These findings revealed that teacher identity differed significantly according to the level of meeting the three basic components of SDT: autonomy, competence, and relatedness. The teacher, who gave her students the right to choose experimental activities, collaborated with science centers and included parents in the process, developed a more flexible, participatory, and supportive structure. The other teacher allowed limited student participation in decision-making processes and created a more control-oriented classroom atmosphere. These results showed the importance of addressing autonomy-supported approaches in science teacher education.