This study attempted to investigate the effect of problem-based learning (PBL) on the environmental attitudes of preservice science teachers. In the study, an experimental design was used with a pre-test and post-test control group. In all, 51 junior preservice science teachers participated in the study. The study was carried out over 10 weeks 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 environmental attitudes inventory. Data were then analysed using PASW Statistics 18 (SPSS Inc.). The findings of the study revealed a statistically significant increase in favour of the experimental group preservice science teachers' environmental attitudes. An increase in environmental attitudes was also found in the control group; however, this increase was not statistically significant. As a result, PBL was found to be more effective than the traditional teaching approach in the development of environmental attitudes in preservice science teachers.