Background: Although professional development (PD) programs often use video, extraneous information during video viewing can distract teachers. We developed a discourse visualization tool, the Classroom Discourse Analyzer (CDA), to support teachers' reflections on classroom discourse in video-based PD workshops. Methods: We used a randomized controlled trial with an embedded case study to examine the efficacy of a year-long video-based PD program using the CDA. Findings: The 24 intervention teachers significantly increased their use of productive talk moves in mathematics classrooms relative to the 22 comparison teachers (Cohen's "d" = 0.67 to 2.35, p <0.05). Moreover, a linear mixed model analysis showed that 882 students of the intervention teachers had significantly higher mathematics achievement scores than the 625 comparison students (Cohen's "d" = 0.24, p <0.05). The case study showed that CDA's multiple representations of classroom discourse and interactive, process-oriented visualizations facilitated the teachers' navigation of classroom video data. Additionally, video and visualizations prompted the teachers to reflect on the data with their peers for evidence-based reasoning and discussion. Contribution: This study demonstrates the efficacy of a video-based PD program for increasing classroom discourse and student learning. It also informs the design of visualizations to enrich video-based PD.