In this research, the teaching of mechanical waves was realized with inquiry-based learning; the aim was to determine the changes in students' conceptual understanding of spring, water, and sound waves. It was designed as action research. The study group comprised 58 upper-secondary school students enrolled in 10th grade at an Anatolian upper-secondary school in Turkey. Data collection tools used in the research were the form for conceptual understanding of mechanical waves, semi-structured interviews, video recordings and photographs, student journals, and hand-made materials. Moreover, researchers designed an action plan that was composed of activities on spring, water, and sound waves. Descriptive and conceptual content analyses were utilized in data analysis. At the end of the research, it was determined that there was a change in students' conceptual understanding of spring, water, and sound waves. Moreover, it was determined that teaching based on inquiry-based learning contributed positively to students' learning. In this respect, it is thought that such applications should become widespread. There should be online training for lecturers which in return would contribute to students' learning.