Empirical research considers teacher collaboration to be an important predictor of outcome variables at the student, teacher, and school level. Principals are responsible for shaping teachers' work environments, and in doing so, they can strengthen and support teacher collaboration. Drawing on social interdependence theory, we hypothesized that teachers' collective efficacy has a mediating effect on the relationship between principal leadership and teacher collaboration. We collected data from 630 teachers in 29 primary and secondary schools in Germany and found, based on structural equation modeling, that principal leadership had a significant indirect effect on teacher collaboration, mediated by teachers' collective efficacy. We discuss the implications of these results for supporting school improvement.