The study investigated the effect of problem-based learning (PBL) on students' achievement in chemistry. Learners' low achievement in Science in South Africa has been a concern to government, stakeholders, school principals and parents over the years as a result of poor teaching techniques, students' attitudes, lack of teaching and learning materials, teachers' pedagogical skills, etc. Several studies, for instance the Monitoring Learner Achievement (MLA) project conducted by UNESCO and UNICEF have shown no improvement in the performance of South African students in Mathematics and Science. Quasi-experimental design was employed for the study. 101 equivalent students were selected for the study using pre-test. The control group was taught with the traditional lecture method whiles the experimental group received instruction with PBL. Independent t-test was used for the analysis. Results showed that there was significant difference (p < 0.05) in chemistry achievement of students between control and the experimental group while there was no significant differences in the before the study. The results show that PBL is an effective way for to teach chemistry so as to improve students' critical thinking and problem solving skills.