School-wide positive behavior interventions and supports (SWPBIS) is a school-wide approach to create a safe and positive school climate. SWPBIS is a framework in which core features and procedures need to be adjusted to its specific school context, referred to as contextual fit. Implementing with fidelity is related to positive outcomes such as a decrease of behavioral problems. Therefore, when adapting SWPBIS to the context, fidelity of implementation needs to be assured. At the introduction of SWPBIS in the Netherlands in 2009, several procedures were adapted to the Dutch educational context, and different modalities of supporting schools in implementing SWPBIS emerged. In this study, the "Tiered Fidelity Inventory" (TFI) and School-Wide Evaluation Tool (SET) were used to assess fidelity of Tier 1 implementation in 117 Dutch schools. The average period of SWPBIS implementation was 2 years 5 months. Results showed that all core features and procedures were present. Mean total scores were 60% for the TFI and 70% for the SET. Most participating schools appeared to have leadership teams, expectations were taught, and acknowledgment provided. Teams had been trained, and discipline data collected. Compared with other features, annual evaluation, data-based decision making, and stakeholder involvement were less well implemented.