Indonesia, like many other countries, see Information and Communication Technology (ICT) as a potential tool for enhancing the quality of education. However, there is a lack of research examining Indonesian secondary mathematics teachers' knowledge in the integration of ICT. In this study, we aimed to investigate Indonesian secondary teachers' knowledge in the use of ICT in secondary mathematics classrooms. The study employed a quantitative method with a cross-sectional survey approach. It was conducted in one of Indonesia's provinces where the data were collected from 341 secondary mathematics teachers through a questionnaire survey. The findings suggest that, to a large extent, Indonesian secondary mathematics teachers have a largely inadequate knowledge of ICT and knowledge of ICT use in teaching. Based on the findings, we argue that it is crucial to improve Indonesian teachers' knowledge of both aspects, and more training courses for teachers' knowledge development are needed.