This article describes the processes employed in developing the Social Justice Teacher Leader Self-Assessment (SJTLSA), a tool designed to foster teacher leader self-reflection, stimulate collegial dialogue, assess school culture, and direct social justice initiatives. Tool development procedures included examining precedents, developing a prototype, establishing face validity and pilot testing. The resulting SJTLSA offers a procedure for systematic self-evaluation of six dimensions of teacher leadership: Dispositions, Knowledge, Personal Attributes, Personal Experiences, Leadership Skills and School Culture. The final format includes 75 closed-response questions, a self-scoring guide, and reflective questions to help respondents identify a personal focus for action research. When used in multiple contexts, the SJTLSA has been found to increase self-awareness, expand knowledge of social justice and teacher leadership and increase respondents' identity and agency as teacher leaders. Through its content and structure, the SJTLSA theorizes the knowledge, attributes and dispositions essential for social justice leaders, and the leadership skills, personal experiences and school cultures that support teacher leaders to be effective as change agents. As a practical tool that stimulates reflection and dialogue, the SJTLSA supports practitioners including informal and formal leaders in school systems, graduate program faculty, and facilitators of professional development and leadership development programs.