BACKGROUND: A walking school bus (WSB) consists of students and adults walking to and from school and promotes active commuting to school. Self-efficacy (SE) and outcome expectations (OE) are behavioral constructs associated with active commuting to school. The authors sought to assess the impact of a WSB program on child SE, and parent SE, and OE. METHODS: The authors conducted a cluster randomized controlled trial of a WSB intervention from 2012 to 2016 among 22 elementary schools serving racially diverse, low-income populations in Houston, TX and Seattle, WA. Surveys collected data from third- to fifth-grade students and their parents, (n = 418) child-parent dyads, before school randomization and at the school year's end. Child surveys included 16 SE items, while parent surveys included 15 SE items and 14 OE items. Scores were averaged from responses ranging from 1 to 3. The authors compared changes in SE and OE between groups over time and accounted for clustering using linear mixed-effects models. RESULTS: The intervention group had increases in child SE of 0.12 points (P = .03), parent SE of 0.11 points (P = .048), and parent OE of 0.09 points (P = .02) compared to controls over time. CONCLUSIONS: As hypothesized, the WSB improved child SE, parent SE, and parent OE related to active commuting to school.