This study revisits the relationship between teachers’ professional knowledge and professional practice by focusing on the role of expectations of the ideal teacher self in the process through which professional knowledge is translated into practice in authentic educational settings. Using data collected from 1,690 secondary school teachers in Gyeonggi Province, multiple regression analyses were conducted. The results indicated that, first, teachers’ professional knowledge had a significant positive effect on professional practice. Second, expectations of the ideal teacher self also exerted a significant positive effect on professional practice. Third, expectations of the ideal teacher self moderated the relationship between professional knowledge and practice, such that the effect of knowledge on practice was strengthened as teachers’ expectations of their ideal teacher self increased. These findings suggest that, alongside the acquisition of professional knowledge, fostering future-oriented expectations of the ideal teacher self plays a critical role in promoting the effective transfer of knowledge into practice within teacher professional development.