Background: Active commuting to school (ACS) is associated with increased physical activity and lowered risk of obesity. In observational studies, ACS was associated with child self-efficacy, parent self-efficacy, and parent outcome expectations, although few experiments have assessed changes in these behavioral constructs. Aim: This study examined the effects of a bicycle train intervention (BTI) on child self-efficacy, parent self-efficacy, and parent outcome expectations in a diverse, low socioeconomic status population. Method: Data were from a 2014 BTI pilot randomized controlled trial (RCT) on fourth to fifth graders aged 9 to 12 years, n = 54, from four schools serving low-income populations in Seattle, Washington. The BTI was a group of children and study staff who cycled together to/from school daily, while controls received no intervention. Responses to validated child self-efficacy, parent self-efficacy, and parent outcome expectations questionnaires ranged from 1 to 3. Adjusted linear mixed effects models estimated standardized coefficients for child self-efficacy, parent self-efficacy, and parent outcome expectations comparing intervention and controls from Time 1 (preintervention) to Time 2 (final 4-6 weeks of intervention). Results: The intervention group had increases in child self-efficacy of 0.84 standard deviations (95% confidence interval [CI] [0.37, 1.31]), parent self-efficacy of 0.46 standard deviations (95% CI [0.05, 0.86]), and parent outcome expectations of 0.47 standard deviations (95% CI [0.17, 0.76]) compared with controls from Times 1 to 2 (all ps <0.05). Conclusion: A BTI improved child self-efficacy, parent self-efficacy, and parent outcome expectations, which warrants a larger RCT to examine long-term changes to these behavioral constructs and ACS.