Education thought leaders highlight the need to expand access to high-quality STEM learning for students historically underrepresented in STEM fields, and project-based learning (PjBL) is often seen as a key strategy. Introducing PjBL to classrooms typically focuses on building teacher capacity, but principals also play a key role in effective implementation. This paper describes findings from an action research study that investigated ways to help school leaders learn to support PjBL in ten mid-high poverty schools as part of a federally funded grant to increase access to STEM for underrepresented students. Findings highlight the need to (a) help principals understand how PjBL can support learning needs by observing implementation with similar student populations, (b) provide principals with a variety of targeted learning opportunities to learn how to support implementation, (c) consider school context when planning PjBL support, and (d) ensure plans for PjBL implementation are aligned with systemwide goals.