Although followers' needs are a central aspect of transformational leadership theory, little is known about their role as mediating mechanisms for this leadership style. The present research thus seeks to integrate and extend theorizing on transformational leadership and self-determination. In particular, we propose that the satisfaction of followers' basic needs (autonomy, competence, and relatedness) mediates the relationship between transformational leadership and employee outcomes (job satisfaction, self-efficacy, and commitment to the leader). We tested this model in two studies involving employees from a broad spectrum of organizations in Germany (N = 410) and in Switzerland (N = 442). Results revealed largely consistent patterns across both studies. The need for competence fulfillment solely mediated the link between transformational leadership and occupational self-efficacy; the need for relatedness fulfillment solely mediated the link between transformational leadership and commitment to the leader. The mediating pattern for the link between transformational leadership and job satisfaction varied slightly across studies. In Study 1, only the need for autonomy fulfillment was a significant mediator, whereas in Study 2, all three needs mediated this relationship. Taken together, our study integrates theorizing on transformational leadership and self-determination by corroborating that need fulfillment indeed is a central mechanism behind transformational leadership.