Context:  The unique contexts in which athletic trainers (ATs) work require specific tools in order to understand their attitudes toward diverse patient populations, including sexual and gender minorities. Objective:  To develop and validate the Attitudes Toward Transgender Patients (ATTP) instrument for ATs. Design:  Cross-sectional study. Setting:  Semistructured interviews, paper-based questionnaire, and electronic questionnaire. Patients or Other Participants:  Six ATs completed semistructured interviews to develop themes regarding transgender patients. Additionally, 39 students in professional and postprofessional athletic training programs answered questionnaires designed to elicit statements regarding transgender patients. For item reduction, a sample of 3000 ATs were e-mailed (response rate = 17%), and for validation, another sample of 3000 ATs were e-mailed (response rate = 13%). Athletic trainers' e-mail addresses were obtained from the National Athletic Trainers' Association. Data Collection and Analysis:  The 3 phases were (1) exploratory interviews, (2) construct validity and item reduction, and (3) criterion validity. Items were created based on interviews and questionnaires. Principal axis factoring was used for item reduction, and Pearson correlations were used for validation. Results:  Thirty-six statements pertaining to transgender patients were developed from the interview and questionnaire data. After item reduction, 10 items remained to form the ATTP (α = .834). For validation, the ATTP and Transphobia Scale were significantly correlated (r = .723; P < .001). Conclusions:  The ability to assess attitudes toward transgender patients will allow clinicians to identify needed areas of focus for training and education. The ATTP assesses affective and cognitive attitudes and behavioral intentions toward transgender patients in common clinical settings.