Since 2013, the Uruguayan educational system has been using an online adaptive learning tool for mathematics: The Mathematics Adaptive Platform (PAM for its Spanish acronym). PAM’s content has been adapted to the national curriculum and it is a tool that - based on an analysis of students' experiences - offers personalized feedback according to each student's skill level. The use of PAM has been spreading throughout the education system. By 2016, approximately half of all students in 3rd through 6th grades of primary education had used the platform. The purpose of this study is to identify the effect of the use of PAM on the test score gain in mathematics based on longitudinal data from a sample of students in primary education. The results show a positive effect of 0.2 standard deviations on mathematics test scores. Results also show that the impact of PAM increases as the socioeconomic status of students decreases. There is no heterogeneous impact by gender. This is the first evidence at a country-wide level of the impact of a pedagogical tool of this type.