This study investigated the mediator role of metacognitive self-regulation strategies in the relationship between students' perceptions of constructivist learning environments and critical thinking dispositions in science. The data were collected through self-report questionnaires from 678 Turkish seventh-grade students. Path analysis showed that students' perceptions of constructivist learning environments and use of metacognitive self-regulation strategies positively predicted their critical thinking dispositions. Moreover, students who reported high levels of constructivist learning environment perceptions also reported using more metacognitive self-regulation strategies. Mediation analysis indicated that metacognitive self-regulation strategies mediated the relationship between constructivist learning environment perceptions and critical thinking dispositions. Findings of the study highlight the importance of constructivist features of the learning environment and metacognitive self-regulation strategies in students' critical thinking dispositions in science.