Teacher advocacy has been examined as a practice of activism external to the school and as a practice of educational leadership. However, researchers have not merged these ideas by framing advocacy as a practice of leadership that takes place within the classroom and across the school. This article illustrates how, through advocacy on behalf of students who are marginalized, teachers demonstrate a model for critical teacher leadership. Using a critical ethnographic approach, observation and interview data were collected across three schools to answer these research questions: (a) In what ways do teachers advocate on behalf of students who have been marginalized; (b) How do these teachers garner support for the students from administrators and/or colleagues; and (c) How do teachers advocate for these students beyond their school? Findings are used to support teacher advocacy as a practice of critical pedagogy, collaborative leadership, and ethical leadership, and to propose a definition of critical teacher leadership.