Universal Design for Learning (UDL) holds considerable promise to create inclusive educational environments. Nevertheless, the most recent theoretical UDL model, which includes both teachers' philosophy and praxis of teaching, has never been tested empirically. Therefore, this study aims to validate the UDL model as a "whole" among preservice teachers. Results show that the three philosophical constructs of UDL predict the performance of preservice teachers' practices associated with UDL. These constructs are growth mindset about learning, self-efficacy to implement inclusion, and self-regulation and motivation for teaching. Results also show that preservice teachers think and reason about UDL not as three separate principles (i.e., engagement, representation, action, and expression) but in an interrelated way as the analysis shows them to be a unidimensional factor. Finally, this article discusses the implications of a validated model on UDL for teacher-educators, practitioners, and researchers.