Drawing upon trait-activation-theory, this study explores the moderating role of resource support in the relationship of middle-leaders' personality traits, namely extraversion, conscientiousness, and openness to collective teacher efficacy. The study used a two-source survey design with data from 609 participants: middle-leaders and teachers in 103 secondary schools in Israel. The findings reveal that resource support served as a moderator in the relationship of the personality traits of extraversion and openness to experience and collective teacher efficacy. Specifically, when resource support was low, negative correlations between middle-leaders' extraversion and openness to experience and collective teacher efficacy were found. However, no correlations were found when resource support was high. No significant influence of resource support was found on the relationship of conscientiousness to collective teacher efficacy. This study adds and deepen our understanding regarding the complex interplay among middle-leaders' personality traits, school resources support and collective teacher efficacy, emphasising the need for adequate resource support to maximise middle-leader traits' influences and to promote collective teacher efficacy. These findings have significant practical implications for teacher and teaching education, teacher development programmes and practices, resource allocation decisions, and professional development initiatives.