Turkey has witnessed numerous changes in the transition to secondary education systems in the 2000s. The Secondary Education Institutions Examination (OKS), the Level Specifying Examination (SBS), and the Transition from Primary to Secondary Education System (TEOG) were implemented between 2010 and 2018. Lastly, in 2018, the Transition to High School System (LGS) has been adopted as a new system. This study examines the effect of the changes in secondary education transition systems between 2010 and 2019 on students' mathematics achievement in 9 th grade. The data of all students who were in 9 th grade between 2010 and 2019 were analyzed through multiple linear regression methods. The parents' education level, 8 th grade mathematics achievement, age, and gender are considered as external variables and their effects are controlled statistically. The findings revealed that a significant and sharp decrease in 9 th grade mathematics achievement occurs after every system-level change in the transition systems. Additionally, the largest decrease in mathematics achievement is found in TEOG. Based on the results, it is recommended to make incremental improvements instead of radical changes to alleviate the problems in secondary education transition systems.