This study aimed to assess the impact of problem-based learning (PBL) on students' achievement in mechanical waves among secondary schools in South Western Uganda. Four hundred and nineteen students (419) from 19 schools were involved in this study. A quasi-experimental research method was employed through Solomon's four-group design. Form six physics students were randomly allocated to the experimental group exposed to PBL and the control group exposed to traditional instructional methods (TIM). A Mechanical Wave Conceptual Survey (MWCS) was administered twice as a pre- and post-test to both groups. Exposition to these instructions lasted three and half months. The study found that students' achievements in waves improved in the PBL than in the TIM learning environment, supported by the large effect size and high learning gains in the experimental group than those in the control group. Specifically, among groups offered both pre- and post-test during Solomon's four-group sampling at the post-test stage, the experimental group achieved higher than the control group. Further analysis among factors such as gender difference, age difference, subjects' combination, single girls alongside mixed schools, and government or private owned schools were analyzed, and practical implications such as teacher's adaptation of PBL were recommended.