Computer science (CS) education is plagued by a gender divide, with few girls and women participating in this high-status discipline. A proven strategy to broaden participation for girls and other underrepresented students interested in CS is the availability of teacher preparation that requires classroom teachers to grow their knowledge of CS content as well as the pedagogical practices that enhance inclusive learning opportunities for historically underrepresented students. This case study describes the design and impact of an Online Professional Development (PD) for CS teachers, a year-long PD program aimed at broadening participation in the United States. Using survey and observation data from more than 200 participants over three years in PD settings, this paper examines how the design of an online learning community model of PD provides an inclusive venue for teachers to examine their belief systems, develop inclusive pedagogical practices, and collectively transform the culture of CS classrooms to places that support all learners. Findings suggest that purposeful facilitation creates a transformative culture of “shared experience” whereby facilitators and groups of teachers engage in collaborative lesson planning and debriefing discussions, in both synchronous and asynchronous sessions. This case study can inform other online PD efforts aimed at broadening participation in computing.