This study aimed to examine the mediating role of teachers’ perceptions of the principal-teacher relationship in the relationship between teachers’ perceptions of innovative school leadership and school happiness. The sample of the study consists of 400 teachers working in state secondary and high schools. Data were collected through the “Innovative School Leadership Scale”, “School Happiness Scale” and “Principal-Teacher Relationship Scale”. Regression analysis based on the Bootstrap method was conducted to determine the mediation in the research model. The first hypothesis in the study is that teachers’ perceptions of innovative school leadership positively affect their perceptions of school happiness. The second hypothesis is that teachers’ perceptions of innovative school leadership positively affect their perceptions of the principal-teacher relationship. The third hypothesis is that teachers’ perceptions of the principal-teacher relationship positively affect their perceptions of school happiness. The fourth hypothesis is that the principal-teacher relationship mediates the relationship between innovative school leadership and school happiness. The hypotheses were confirmed. The research provides valuable information in both theoretical and practical terms for developing strategies to increase school happiness. At the end of the research, limitations and suggestions for future studies are included.