Ultraprocessed food (UPF) consumption is associated with noncommunicable disease risk, yet no trial has assessed its health impact within the context of national dietary guidelines. In a 2 × 2 crossover randomized controlled feeding trial, 55 adults in England (body mass index ≥25 to <40 kg m ⁻² , habitual UPF intake ≥50% kcal day ⁻¹ ) were provided with two 8-week ad libitum diets following the UK Eatwell Guide: (1) minimally processed food (MPF) and (2) UPF, in a random order. Twenty-eight people were randomized to MPF then UPF, and 27 to UPF then MPF; 50 participants comprised the intention-to-treat sample. The primary outcome was the within-participant difference in percent weight change (%WC) between diets, from baseline to week 8. Participants were blinded to the primary outcome. MPF (%WC, −2.06 (95% confidence interval (CI), −2.99, −1.13) and UPF (%WC, −1.05 (95% CI, −1.98, −0.13)) resulted in weight loss, with significantly greater %WC on MPF (Δ%WC, −1.01 (95% CI, −1.87, −0.14), P = 0.024; Cohen’s d , −0.48 (95% CI, −0.91, −0.06)). Mild gastrointestinal adverse events were common on both diets. Findings indicate greater weight loss on MPF than UPF diets and needing dietary guidance on food processing in addition to existing recommendations. Clinicaltrials.gov registration: NCT05627570 .