This study aimed to investigate the types of adaptations the teacher makes to speaking teaching materials in an English textbook, as well as the factors influencing the teacher’s decisions to make such adaptations during the learning process. This study employed a qualitative case study design. The participant was an English teacher at a public high school in Surakarta. Research data were collected through guided written reflections, semi-structured interview, classroom observation, and document analysis. Data were analyzed using interactive analysis techniques, including data reduction, data display, and conclusion drawing. The findings indicate that the teacher actively adapts speaking teaching materials, and the most common adaptations involve modifying activities, adding achievable challenges, and simplifying activities. In addition, the findings indicate that teachers’ decisions regarding adaptations are most influenced by student-related factors and time constraints. The result also indicates that the teacher flexibly adapts speaking teaching materials to ensure that the curriculum goals are met. Based on these findings, English teachers are advised to be more flexible and responsive to students’ needs and learning conditions. Furthermore, future research could explore the adaptation of teaching materials for other language skills, such as listening, reading, and writing.