The prevalence of systems thinking pedagogy is growing in classrooms because of its inclusion in the Next Generation Science Standards and its potential as a tool for addressing complex, global problems. While most of the research on systems thinking targets the ways students develop system thinking skills, this qualitative study explores teachers' construction of systems thinking as a pedagogy in a professional development context. With this paradigm-shifting pedagogy, teacher learning in professional development is a critical piece of understanding how the practice is taken up. Using social constructionism, discourse analysis, and systems theory, we conducted a holistic case study to analyze how teachers privileged language and ways of knowing during a professional development institute on systems thinking. We found that the teachers considered systems thinking as a catalyst for shifting their thinking, curriculum, and classroom dynamics; imperative to this shift was the implementation of systems mapping as the central activity of the pedagogy. Their model of systems thinking pedagogy respected both teachers and students as capable academics; additionally, it privileged a professional development culture of "becoming" in which the power of the collective was a point of leverage for teachers to disrupt what can be perceived as low expectations for their students and themselves as professionals.