There is a rich literature about the benefits of the Positive Behavioral Interventions and Supports (PBIS) for schools. However, evidence on teacher-related outcomes is relatively limited. The current study examines the effect of the PBIS Tier 1 implementation on collective teacher efficacy (CTE). Participants were 530 teachers from 60 primary schools in Cyprus and Greece, who completed the Collective Teacher Efficacy Belief Scale (CTEBS) before and after a 1-year PBIS implementation. The Greek language version of the instrument was tested and validated. Results were in line with the original two-dimensional structure of instructional strategies and behavioral management, with adequate internal consistency ([omega] = 0.89 and [omega] = 0.87, respectively). Multilevel analyses showed that premeasurement and Tier 1 implementation fidelity along with selected teachers' demographic characteristics accounted for a considerable amount of variance of CTE's both dimensions (30.84% and 25.43% for instructional strategies and behavioral management, respectively). The results suggest that PBIS Tier 1 training and subsequent implementation contributes to teachers' collective belief that they can provide quality teaching opportunities and manage unexpected behaviors as a team. Schools are encouraged to consider the application of whole-school approaches for improvement and actively engage all school staff in their efforts. Implications for practice and further research are discussed.