This study investigated the comparative effectiveness of problem-based learning (PBL) as an inquiry-based learning (IBL) in enhancing retention of concepts, problem-solving skills, motivation, and interest in learning among elementary students in Calabar Municipality. The study adopted a quasi-experimental research design, specifically a pretest-posttest control group design. A sample of 300 students was selected using a stratified sampling technique from a population of 3,850 secondary school students in the research area. Data analysis was conducted using population t-test to compare the effects of PBL on student outcomes. One research hypothesis was formulated to guide the study. The first hypothesis posited that students taught using PBL as an inquiry-based learning would not exhibit better retention of concepts and enhanced problem-solving skills compared to those taught using traditional methods. A validated questionnaire tagged "Project-Based Learning Questionnaire" was used for data collection, with a reliability coefficient of 0.85-0.90. The findings indicated that students taught using PBL showed significantly better retention of concepts and problem-solving skills. Based on these findings, it is recommended that elementary school teachers in Calabar Municipality adopt PBL to enhance student learning outcomes.