This study aimed to determine the effect of classroom learning environment on students' academic performance in Mathematics subject in public day Schools in Rwanda: A case of Musanze district. The study has a correlational research design, with both quantitative and qualitative research. The target population of the study consisted of education officers in the area, including students, teachers and school headmasters. Therefore, the total number of population was 1,600. A sample size of 320 respondents was selected using random sampling techniques. Quantitative data were analyzed using descriptive statistics. Qualitative data were analyzed using content analysis. Data analysis was supported by Statistical Package for the Social Sciences (SPSS) version 21. Pearson correlation analysis showed that the classroom learning environment (r=0.714, p=0.000) was positively and significantly correlated with students' academic performance. This correlation was also statistically significant as the p-value was less than 5%. In conclusion, the study provided recommendations to improve students' academic performance. The study recommends that the management of schools should frequently visit classrooms to assess participation, involvement, and understanding of the students and offer support to those teachers who lack the competencies required to improve the learning situation and environment in mathematics classrooms due to the size of the class.