The purpose of this study was to investigate the effect of problem-based learning (PBL) on the metacognitive awareness of pre-service science teachers. In the study, an experimental design with pre-test/post-test control group was used. A total of 51 junior pre-service science teachers participated in the study. The study was carried out over 10 weeks and within the scope of an environmental science course. During the study, lessons in the experimental group were processed using a PBL approach while lessons in the control group were processed using a traditional teaching approach. Data were collected through a personal information form and Metacognitive Awareness Inventory. Data were then analysed using PASW Statistics 18 (SPSS Inc.). The findings of the study revealed that PBL could be an effective intervention to promote metacognitive awareness towards procedural knowledge, planning and debugging. The results are discussed based on the findings of the study.