Background: Academic performance in school stems from an interaction of factors associated with students, families and schools. Among these factors, physical activity could play a very relevant role. Objective: The goal of this study was to determine whether students' physical activities in and outside school were related to their academic performance. Design, setting and method: We used the Programme for International Student Assessment (PISA) 2015 database for Spain using ordinary least squares (OLS) regression models to answer our research questions. Results: Results suggest a positive association between the number of days per week doing moderate physical activities and academic performance in science, reading and mathematics. However, a higher number of days per week doing vigorous physical activities was associated with lower scores in reading and science. We also find a negative association between exercising or practising sports before going to school and the scores achieved in the three competences evaluated. Conclusions: Our results show that physical activity can play an important role in academic performance and that the effects of physical activity on children and teenagers should be further investigated.