Increasing demand for high-quality STEM education calls for the formation and retention of effective educators with STEM expertise; yet endeavors to meet this demand require organizational structures and supports that can overcome the lack of collective understanding around STEM education, limited STEM knowledge by leadership, and mounting teacher attrition. STEM teacher leadership is one mechanism for addressing this challenge by empowering excellent classroom teachers with opportunities to institutionally shape STEM learning, without extracting them from their instructional role. This paper offers a STEM teacher leadership framework that targets teachers' identity development toward an equity-focused, transformational STEM teacher leader. First, we examine the field of teacher leadership (both STEM-specific and generally) to provide a foundation for the areas of potential impact by STEM teacher leaders, as well as dispositions and stances that support their emergent leadership. Complementing the literature, we explore and elevate aspiring STEM teacher leaders' perspectives, drawn from baseline surveys and interviews of a multi-year professional development program for middle grade teachers in STEM disciplines. The framework, iteratively informed by these sources, includes our stance on equity in STEM education, eight dispositions, and four domains of knowledge and practices that shape STEM teacher leadership in K-8 school settings.