Background: Highly processed foods are inexpensive and abundant in our food supply, nutritionally poor, and disproportionately marketed to minority youth. This study is part of a curriculum development project to develop, implement, and evaluate the "In Defense of Food" (IDOF) curriculum designed to increase intake of whole/minimally processed foods and decrease intake of highly processed foods in youth. Aims: This pilot outcome evaluation was undertaken to assess initial effectiveness and to provide an in-depth understanding of changes in behavioral outcomes and psychosocial mediators. Methods: We used an explanatory mixed method approach, including a single-arm pretest-posttest of intervention effect, followed by a food rules assessment and in-depth interviews to describe participant responses to the intervention in more detail. The study was conducted in three afterschool classrooms in urban low-income neighborhoods with 32 multiethnic middle-school youth, receiving 10 weekly 2-hour sessions. Results: Two weeks postintervention, there was a large positive significant increase in whole/minimally processed food intake (p < 0.01; d = 0.59) and a small decrease in consumption of highly-processed foods (p = ns; d = 0.06), compared with baseline. Significant increases in psychosocial mediators: Self-efficacy and positive outcomes expectations were seen; others were not significant but changed in the desired direction. Qualitative assessments suggest that the intervention promoted skill building, but environmental barriers made these difficult to use. Discussion: The IDOF curriculum may be most effective for promoting consumption of fruits and vegetables, rather than decreasing intake of highly processed foods. In addition, in this young age-group, short actionable food rules may support self-regulation and behavior change. Conclusion: Among adolescent students in low-income urban neighborhoods, the IDOF afterschool curriculum may help promote self-efficacy and positive outcome expectancies and increase fruit and vegetable intake. Focusing on food processing and using "Food Rules" may be promising to elicit behavior change in youth; however, greater supports are needed to overcome social and environmental barriers.