This study examined the relationships of teachers' work engagement with their professionalism and openness to change and investigated the moderating role of the transformational leadership of principals in explaining these relationships. Multilevel data were collected from 1,886 teachers from 59 schools in South Korea, and multilevel path analysis was performed to analyze the data. The study results found that teachers' work engagement was positively influenced by their professionalism and openness to change in the Korean educational setting. However, no significant relationship was found between teachers' work engagement and the transformational leadership of principals. It also showed that the transformational leadership of principals positively moderated the relationship between work engagement and openness to change of teachers. Finally, we identified the negative moderating effect of the transformational leadership of principals on the relationship between work engagement and professionalism of teachers. The theoretical implications of these findings are provided along with practical suggestions for policy makers, superintendents, and teacher education institutes.