Lesson study provides collaborative platforms for enhancing mathematics teachers' knowledge and pedagogical skills. Research on the collaborative learning of professional programs is well documented and specifies that it can support mathematics teachers in transferring their pedagogies, knowledge, and attitude about teaching mathematics to improve students' achievements. This study examined the role of lesson study projects in developing collaborative inquiry skills among high school mathematics teachers in Saudi Arabia, as well as the impact of lesson study projects on students' mathematical learning. This study implemented a scale with certain design features to evaluate the level of collaborative inquiry skills of two groups of high school teachers: Those who attend lesson study projects and those who never do. The study analysis uncovered that high school mathematics teachers who engage in lesson study professional development projects demonstrate higher collaborative inquiry skills than those who never engage in such professional development experiences. A significant finding of this study determined that lesson study experience had a positive impact on the improvement of students' mathematics learning.