As schools increasingly implement multitiered systems of support, there is a pressing need to develop psychometrically sound implementation fidelity measures. The interconnected systems framework (ISF) is a multitiered model blending systems of positive behavioral interventions and supports with promotion, prevention, and intervention strategies of school mental health. The ISF is being implemented in communities across the United States with ongoing evaluation in several randomized controlled trials. The ISF-Implementation Inventory (ISF-II) was developed to measure fidelity of the ISF within a school building. We conducted a national validation study including completion of the ISF-II by 398 educators in 49 schools, 16 school districts, and 9 states. Results indicate the ISF-II produces scores that are internally consistent and structurally valid when items are organized into a three-tiered model. Additionally, the ISF-II was rated as feasible, acceptable, and beneficial. Limitations of the study, including the need for additional psychometric testing, are discussed in light of these results that suggest educators and researchers, alike, should feel confident in using the ISF-II as a measure of ISF implementation quality.