This article discusses a qualitative case study involving eight mathematics teachers in Brazil. The study aimed to analyze the effectiveness of a training course that combined Lesson Study and Criteria of Didactical Suitability in developing reflective competence in teachers. Lesson Study is a professional development strategy that encourages teachers to collaborate and reflect on their teaching practices, while Criteria of Didactical Suitability is a tool that guides teachers’ reflection. The study aimed to evaluate the usefulness of Criteria of Didactical Suitability in enhancing reflection skills and to assess the participants’ perception of its usefulness. The analysis revealed that participating teachers improved their reflection skills, specifically by utilizing Criteria of Didactical Suitability more effectively in the third phase of the course. However, the study has limitations, such as being conducted virtually and in a specific context. Overall, this study highlights the potential of combining Lesson Study and Criteria of Didactical Suitability in developing reflective competence in teachers. Further research can explore the efficacy of this approach in different contexts.