This study examined the mediating role of teachers' professional development activities in the relationship between instructional leadership and collective teacher efficacy. Using a convenience sampling approach, the sample comprised 251 primary, secondary, and high school teachers during the 2022-2023 academic year in Türkiye. The results showed that teachers' views on instructional leadership, collective efficacy, and professional learning activities were above the average. There were statistically significant associations among instructional leadership, collective teacher efficacy, and teachers' professional learning activities. Instructional leadership and teachers' professional development activities significantly affected collective teacher efficacy. Additionally, teachers' professional development activities significantly affected collective teacher efficacy and mediated the association between instructional leadership and collective teacher efficacy. The results suggest that administrators can enhance teachers' collective efficacy by fostering their professional learning when they demonstrate instructional leadership behaviors.