This diary study investigated whether and how supervisors' leadership style influences followers' daily work engagement. On the basis of leadership theories and the job demands–resources model we predicted that a transformational leadership style enhances employees' work engagement through the mediation of self-efficacy and optimism on a day-to-day basis. Forty-two employees first filled in a general questionnaire and then a diary survey over five consecutive workdays. The results of multilevel analyses offered partial support for our hypotheses. Daily transformational leadership related positively to employees' daily engagement and day-levels of optimism fully mediated this relationship. However daily self-efficacy did not act as a mediator. These findings expand theory and previous research by illuminating the role of transformational leaders in fostering employee work engagement.