Introduction <p>Adaptive expertise is a critical outcome of health sciences education, enabling learners to respond effectively to novel and complex clinical situations. The Master Adaptive Learner (MAL) model was developed to foster adaptive expertise across health professions; however, its implementation has been limited by the lack of assessment tools focused on observable learner behaviors. The purpose of this study was to develop the Master Adaptive Learner Behavioral Assessment Scale (MAL BAS) and to provide initial validity, feasibility, and acceptability evidence for its use in health sciences education, beginning with physical therapist education as the application context.</p> Methods <p>This multi-phase study included a comprehensive literature review, a reactive Delphi process with MAL experts, cognitive interviews, and feasibility and acceptability surveys. Data were collected virtually from stakeholders across the United States, primarily physical therapist educators.</p> Results <p>Eight MAL experts from medicine and physical therapy completed two rounds of the reactive Delphi process, and six physical therapist educators participated in cognitive interviews, yielding validity evidence based on test content and response processes. Additionally, 40 physical therapist educators completed feasibility and acceptability testing. The resulting 30-item MAL BAS demonstrated positive feasibility and acceptability, with an average completion time of approximately 11&#xa0;min, and showed utility for use in both academic and clinical education settings.</p> Conclusion <p>The MAL BAS shows promise as a formative assessment tool to support the development of adaptive expertise in health sciences education, with demonstrated utility in physical therapist education. Further research is needed to collect additional validity and reliability evidence.</p>