This classroom action research was aimed at describing the improvement of students' mathematical problem solving (MPS) ability through Realistic Mathematics Education (RME). Nineteen students of VIII grade in SMP Laboratorium Undiksha in the first semester of the academic year of 2019/2020 were taken part as the research subject. This research data was collected by observation sheet and essay-test then it analyzed descriptively using simple statistics. The result of the research showed that the average score of students’ MPS ability on the first cycle equals 71,92, the second cycle equals 73,2, and the third cycle equals 75. As demonstrated, the implementation of RME could increase students’ MPS ability. Hence, the implementation of RME made students more motivated, more active on discussion and learning process, solved problems more easily, and more trained to communicate their understanding.