This study was conducted to determine the relationship between the content knowledge of teachers and the academic achievement in mathematics of Indigenous Peoples (IP) learners in the public elementary schools of Matanao II District, Davao del Sur. Grounded in Mishara’s theory on content knowledge and Benbow's views on academic achievement Utilizing a non-experimental descriptive-correlational research design, data were gathered from 52 mathematics teachers and 280 IP learners through a researcher-made teacher questionnaire and a standardized mathematics test. Quantitative data analysis involved the use of mean and Pearson's correlation coefficient. Findings revealed that teachers exhibited very high content knowledge in planning and assessing learning and high knowledge in facilitating learning. IP learners' performance in mathematics was found to be at the "Approaching Proficiency" level. A significant positive correlation was identified between the teachers' content knowledge and the mathematics achievement of IP learners (r = 0.667, p < 0.05). Specifically, all three dimensions of content knowledge were individually and collectively associated with learners' performance, indicating that greater teacher expertise leads to improved student outcomes. It was concluded that strengthening teachers’ content knowledge—especially in the context of culturally diverse classrooms—is essential for improving the academic performance of IP learners.