Leadership can impact teachers’ motivation, which in turn influences their motivating styles. Teachers’ motivating styles affect the academic achievement, motivation, and well-being of students, especially those who are weaker. However, it is not known if leadership affects motivating styles directly and whether this relationship is mediated by basic psychological needs. This study investigated whether school leadership styles affect teachers’ motivating styles via teachers’ basic psychological needs. Thirty-two teachers from five Singapore secondary schools with lower cut-off results in the Primary School Leaving Examinations (PSLE) completed questionnaires, and half of them were interviewed. Transformational leadership styles, of both principals and reporting officers, were found to predict teachers’ autonomy-supportive motivating style. Satisfaction of the basic psychological need for competence fully mediated the relationship between principals’ transformational leadership and autonomy-supportive motivating style. The qualitative analyses identified specific characteristics of transformational leadership style that facilitated teachers’ autonomy-supportive motivating style and satisfied their psychological needs.