Objective: The average American consumes 55% of their daily energy from ultraprocessed foods (UPF) created through industrial processes and additives not used at home. We investigated if a high-UPF diet alters brain response to milkshake compared with a diet free-from UPF (NonUPF) in emerging adults, who are in a critical period for brain development and typically consume high amounts of UPF. Methods: In a randomized controlled crossover trial participants aged 18-25 completed two, 2-week controlled feeding periods including a UPF (81% UPF) and nonUPF (0% UPF) diet. Before and after each diet intervention participants consumed milkshake concomitant with functional magnetic resonance imaging. Results: In the entire cohort, there were no differences between diet conditions in brain response. An exploratory analysis revealed orbitofrontal cortex (OFC) response to milkshake decreased after the UPF diet and increased following the NonUPF diet in adolescents (18-21 years) but not young adults (22-25 years). Habitual UPF intake (gs) was positively associated with OFC response to milkshake independent of diet intervention in all participants. Conclusions: An acute UPF dietary intervention may only alter brain response in adolescents. Further work is needed to determine potential vulnerability of adolescents to changes in dietary UPF on brain response to rewards.