Rooted in social constructivist learning theory, problem-based learning (PBL) is a tool that deepens students’ learning of complex subjects and improves students’ attitudes and beliefs towards learning. Physics is a subject that students themselves view as challenging. When taking physics, students develop negative beliefs about their own learning of the subject. There is a call for more innovation in the subject area of physics. This study addresses the following: (1) What is the effect of PBL on the achievement of 10th-grade students in mechanics when compared to traditional instruction? (2) How do students’ beliefs and attitudes towards physics change before and after the intervention, and how might these beliefs and attitudes relate to their competency outcomes? The sample of this study comprised 63 students in the 10th grade in a public (non-elite) school distributed across four classes, where the teacher used PBL with the experimental group and traditional teaching with the control group. The physics teacher who implemented PBL has 13 years of teaching experience. The two instruments used were the Force Concept Inventory and the Colorado Learning Attitudes About Science Survey. The result of this study revealed that, although students’ knowledge of physics increased when their teachers adopted the PBL approach, there were no significant changes in their attitudes and beliefs towards learning physics. The implications suggest that there is potential for PBL to be taken up by high school science teachers to improve their students’ physics knowledge, but may this not impact their attitudes and beliefs, which presents questions to investigate further.