The unsatisfactory academic performance of students in Nigerian secondary schools has been blamed on the quality of teachers, and professional development has been identified as a remedy. Hence, this study examined the effects of teacher collaboration as a form of professional development on students' academic achievement in mathematics. The research adopted a pretest-posttest-control group quasi-experimental design involving a 2x2 factorial matrix. The population comprised all the mathematics teachers and students of junior secondary school 1 (JSS 1) in the nineteen (19) public junior secondary schools in Ijebu-North Local Government Area of Ogun State. The sample consisted of one hundred and thirty-five (135) students and five mathematics teachers from the two junior secondary schools purposively selected for the study. Mathematics Achievement Test (MAT), a researcher-developed instrument with a reliability coefficient of 0.86 was used to collect data. The collected data were analyzed using both descriptive and inferential statistics. The results indicated students taught by the teacher that collaborated performed significantly better than those taught by the isolated teacher. Also, the findings indicated no significant effect of age on students' academic achievement in Mathematics. Additionally, the outcomes revealed no significant interaction effect of teacher's group and students' age on academic achievement in Mathematics. The results of this research imply that collaboration among teachers can account for an improvement in students learning. Therefore, school authorities should restructure the school hours to cater for teacher collaboration activities. Also, workshops should be organised for teachers on the importance of teacher collaboration as a means of professional development.