Supermarkets located near schools influence adolescents’ food consumption. The aims of this study were (1) to measure dietary behaviors during school hours, (2) to investigate the effect of a nutrition peer-education intervention in supermarkets within walking distance to secondary schools on nutritional knowledge and attitudes toward healthy eating, and (3) to assess how the intervention was appraised by adolescents with a lower education level. The participants were adolescents aged 12 to 14 years from four secondary schools in Amsterdam, the Netherlands ( n = 432). Cross-sectional analyses were performed to establish dietary behaviors (pretest). A quasi-experimental pre–post design with a comparison school was used ( n = 3 intervention, n = 1 comparison). Intervention schools received the intervention in a supermarket near their school. The comparison school received no intervention. The appraisal of the intervention was assessed in the intervention schools (posttest). Most of the adolescents who purchased foods from retail food outlets near the school (71.1%) did so from supermarkets (88.6%). The nutritional knowledge scores (β = 0.69, 95% CI [0.23, 1.15], p = .003) as well as the attitudes toward healthy eating (β = 0.17, 95% CI [0.04, 0.29], p = .009) of adolescents from the intervention schools were statistically significantly higher after the intervention, relative to the comparison school. Nutrition peer education in supermarkets can improve nutritional knowledge and attitudes toward healthy eating among adolescents with a lower education. Future research on the short- and long-term effects of nutrition peer education on food purchases and dietary intake of adolescents is needed.