This study elucidates the relationship between instructional leadership and teacher self-efficacy by identifying the moderating role of teacher collegiality. Survey data were collected from 1,498 teachers at 53 middle schools in Henan, China. Findings indicate that instructional leadership has a positive impact on teacher self-efficacy. More importantly, teacher collegiality was found to moderate the relationship between principal instructional leadership and teacher self-efficacy. More specifically, when there was a high level of teacher collegiality, instructional leadership positively predicted teacher self-efficacy; however, when there was a low level of teacher collegiality, instructional leadership barely predicted teacher self-efficacy. In addition to elaborating on the relationship between principal instructional leadership and teacher self-efficacy, this study underscores the importance of teacher collegiality in promoting the effectiveness of instructional leadership. How teacher collegiality works on principal instructional leadership impactAn increasing number of studies have examined whether and how principal instructional leadership influences teacher attitudes. However, the impact of principal instructional leadership on teacher collegiality and self-efficacy has not been explored simultaneously. This study investigated a moderation model of principal instructional leadership and teacher self-efficacy. Survey data were collected from 1,498 teachers at 53 middle schools in Henan, China. The results showed principal instructional leadership has a positive impact on teacher self-efficacy. More importantly, teacher collegiality moderated the relationship between principal instructional leadership and teacher self-efficacy. When there was a high level of teacher collegiality, instructional leadership positively predicted teacher self-efficacy; however, when there was a low level of teacher collegiality, instructional leadership barely predicted teacher self-efficacy. This study extended the relationship between principal instructional leadership and teacher self-efficacy and emphasizes the importance of teacher collegiality in promoting the effectiveness of instructional leadership.