Aim: The main purpose of this study was to investigate the impact of teacher's pedagogical content knowledge (TPCK) on students' academic performance in circle theorem. Methods: The study used a mixed-methods research approach and a sequential explanatory design with a sample 210 students selected through the probability systematic sampling technique. The primary data was collected using questionnaire, an interview guide and a circle theorem achievement test. The data collected was analyzed using, the regression tool, and deductive manual thematic analysis, which was used for only the qualitative data collected from the interviews. Results: The study found a significant relationship between the independent variable teacher's pedagogical content knowledge and the dependent variable academic performance in circle theorem signifying that students' performance in circles theorem depends on the pedagogical content knowledge of the teacher. Conclusion: Based on the findings that emanated from the data analysed, the study concluded that teachers' pedagogical content knowledge has a significant relationship with students' academic performance in circle theorem. Recommendation: the study recommends that Ghana Education Service organise training conferences and workshops aimed at improving teachers' pedagogical circle theorem content knowledge.