This study was carried out to determine the effect of guided inquiry teaching method (GITM) on secondary school students' achievement in light and sound waves in Abuja, Nigeria. A sample of 95 students drawn from a population of 1,832 was used for the study. Two research questions were raised and answered in the study using mean and standard deviation while two hypotheses formulated were tested using ANCOVA at 0.05 level of significant. Quasi-experimental research design comprising experimental and control groups was used for the study. The instrument for data gathering was light and sound waves achievement test (LASWAT). A validity index of 0.86 and a reliability index of 0.87 were respectively obtained for LASWAT. Students in both groups were given pre-test before instruction. During the instruction, the same content was presented to the two groups for a period of six weeks and thereafter a post-test was administered. The findings of the study revealed that the experimental group significantly scored higher in post-test than the control group. This is evidenced in H01, F(1, 93) = 85.476 and P = 0.000 < x = 0.05. The results also showed that gender has no significant influence on mean achievement scores of students taught light and sound waves using GITM as revealed in H02, F(1,48) = 0.656 and P = 0.413 > x = 0.05. The findings simply imply that GITM is superior to the conventional method in enhancing students' achievement in light and sound waves irrespective of gender in Abuja. Nigeria.