Adaptive teaching has become increasingly important in research and practice. However, its complexity calls for effective professional development approaches, such as Lesson Study, that may promote adaptive teaching practices. This study uses a quasi-experimental mixed-methods design, consisting of stimulated recall interviews and classroom observation instruments, to determine whether participating in Lesson Study leads to more adaptive teaching practices in both teachers’ perceptions and behavior. Although intervention group teachers reported various important changes in their perceptions about adaptive teaching and their (adaptive) teaching behavior, no intervention effects were found. This raises questions about how adaptive teaching can be defined and measured.