As it stands, there are few measurement tools for leaders to use in assessing the degree to which they are supporting the growth of teacher leadership within their school, and those that do exist are based on varying, and sometimes conflicting, definitions of teacher leadership. Furthermore, principals also need to know whether their teacher leadership initiatives are supporting or thwarting teacher motivation. Using the theoretical framework of self-determination theory, this dissertation advances principal support for the development of teacher leadership (PSDTL) as a concept and tool to measure teacher perception of a principal’s efforts to develop teacher leadership within a school. By surveying teachers in a large urban midwestern school district, PSDTL was conceptualized, designed, and tested as a new instrument intended to measure just such principal practice—the degree to which principals were perceived as supporting teacher leadership development via support for teachers’ psychological needs as learners. PSDTL was found to have strong reliability and reasonable validity. Additionally, PSDTL was positively associated with a school’s faculty trust in principal and collective teacher efficacy, suggesting PSDTL is related to the conditions of effective leadership and school improvement.