District leadership teams perform key roles in building the systems to support schools in the implementation of Positive Behavioral Interventions and Supports (PBIS). However, there is a lack of measures for assessment and progress monitoring specific to district PBIS systems. To address this gap, we evaluated the validity of a measure of implementation of district PBIS systems, the "District Systems Fidelity Inventory" (DSFI). Using 183 school districts and 760 schools implementing PBIS, we found the DSFI to have good evidence of structural validity for measuring nine aspects of district systems (Leadership Teaming, Stakeholder Engagement, Funding and Alignment, Policy, Workforce Capacity, Training, Coaching, Evaluation, and Local Implementation Demonstrations). We also found DSFI subscales to be moderately related to school-level PBIS implementation fidelity, providing evidence of convergent validity. We describe how leadership teams can use the DSFI to improve PBIS implementation and student outcomes. [For the corresponding grantee submission, see ED626952.]