Teacher professional agency refers to the ability of teachers to control their work within structural constraints. In this paper, we show how teacher activist organizations can assist in the development of professional agency. We focus on a teacher activist organization in a large urban district in the United States and identify three organizational processes. We find that teacher activist organizations can facilitate efforts to alter classroom instruction, create opportunities for teachers to challenge educational structures, and foster a shared belief in the collective power of teachers. However, teacher activist organizations may be limited in scope and impact, tempering these potential benefits.