Full and durable implementation of school-based interventions is supported by regular evaluation of fidelity of implementation. Multiple assessments have been developed to evaluate the extent to which schools are applying the core features of school-wide positive behavioral interventions and supports (SWPBIS). The SWPBIS Tiered Fidelity Inventory (TFI) was developed to be used as an initial assessment to determine the extent to which a school is using (or needs) SWPBIS, a measure of SWPBIS fidelity of implementation at all three tiers of support, and a tool to guide action planning for further implementation efforts. In this research, we evaluated the psychometric properties of the TFI in three studies: a content validity study, a usability and reliability study, and a large-scale validation study. Results showed strong construct validity for assessing fidelity at all three tiers, strong interrater and 2-week test–retest reliability, high usability for action planning, and strong relations with existing SWPBIS fidelity measures. Implications for accurate evaluation planning are discussed.