Objective: The objective of this research is to study effects of a 4-week high-protein (HP) diet on energy intake, resting energy expenditure (REE), protein turnover and body composition in children with obesity. Methods: In this randomized placebo-controlled single-blind crossover study, children with obesity (n = 14; mean age: 10.1 years ± 1.2 standard deviation; body mass index–standard deviation score [BMI-SDS]: 2.8 ± 0.5) received an ad libitum HP (+50 g protein per day) or normal-protein (NP) diet for 4 weeks with a washout period of ≥2 weeks. Energy intake, REE, protein turnover, weight, BMI-SDS and body composition were measured. Results: No differences were found in energy intake or REE between HP and NP. There was an increased urea production and phenylalanine hydroxylation after HP compared with NP (p < 0.05). There was an increased rise in fat-free mass after HP compared with NP (∆HP: 0.8 ± 0.8 kg vs. ∆NP: 0.1 ± 0.6 kg, p < 0.05). BMI and BMI-SDS increased during the study (BMI-SDS start: 2.8 ± 0.5, end: 2.9 ± 0.5, p < 0.05) without a difference between groups. Conclusions: A 4-week HP diet with ad libitum food intake did not affect energy intake and energy expenditure in children with obesity. BMI increased, although that could be partly explained by an increase in fat-free mass.