Context: Education of athletic training students should include the acquisition of soft skills. Evaluation of soft skills is important but challenging. Multisource performance reviews can provide students with feedback from different viewpoints, which can be used to promote self-awareness and encourage reflection. Objective: To develop and validate a new multisource soft skills assessment tool for master's level professional athletic training students. Design: Cross-sectional study. Setting: Web-based questionnaires. Patients or Other Participants: Nineteen participants evaluated the instrument for face and content validity. These participants included 5 athletic training educators, 7 athletic training preceptors, and 7 students currently enrolled in a graduate-level, Commission on Accreditation of Athletic Training Education-accredited athletic training education program. Participants in the first pilot test included 22 students, 22 preceptors, and 3 instructors. Participants in the second pilot test included 28 students, 32 preceptors, and 3 instructors. Data Collection and Analysis: A 3-stage process was used to develop and validate the instrument. The process included a literature review and formation of 65 survey items organized into 11 themes. Questions were reviewed by students, preceptors, and instructors. Responses were analyzed using content validity ratio to help identify items needing revision, deletion, or substitution. All open responses were considered. Based on results, 1 question was removed, and 2 questions were updated to improve clarity. The updated Athletic Training Student--Soft Skills Assessment instrument includes 64 questions. In the final stage, reliability analysis was conducted on 2 samples. Internal consistency was examined with Cronbach [alpha] and interitem correlations. Results: Using a rigorous content validation approach provided evidence that the instrument was comprehensive, clear, and complete enough to establish the tool's credibility in the preliminary stages. Conclusion(s): The outcome of this study is a multisource soft skills assessment tool that can provide students with formative feedback.