ABSTRACT Objective This study examined the impact of a 2‐week eucaloric diet high in ultraprocessed foods (UPF) compared to a diet without UPF (non‐UPF) on ad libitum energy intake (EI) and food selection in individuals aged 18–25. Methods In a randomized, crossover, proof‐of‐concept trial, participants completed two 14‐day controlled feeding periods (81% UPF vs. 0% UPF), with a 4‐week washout. Diets were matched for macronutrients, fiber, added sugar, diet quality, and energy density. Following each condition, participants consumed an ad libitum buffet meal including UPF and non‐UPF. Energy and food grams consumed were quantified. Statistical analyses were conducted for the full sample, late adolescents (aged 18–21), and young adults (aged 22–25). Results Twenty‐seven individuals aged 22 ± 2 years (mean BMI = 24 ± 3 kg/m 2 ) were included. Diet compliance was ~99% overall. There was no effect of diet condition on meal total kcal or grams consumed or UPF or non‐UPF consumption in the full sample (all p > 0.05). In the exploratory age subgroup analysis, an interaction between diet and age was observed for total EI ( p < 0.001), where total EI increased among adolescents following the UPF diet ( p = 0.03, d = 0.79), but not in young adults. Conclusions Late adolescents may be susceptible to increased EI following a UPF diet. Future trials are warranted to evaluate this possibility. Trial Registration: ClinicalTrials.gov : NCT05550818.