Introduction: Attention-Deficit/Hyperactivity Disorder (ADHD) is a frequent neurodevelopmental condition in childhood. Although pharmacological treatments are effective for many patients, a substantial proportion show partial response or experience adverse effects. Nutritional strategies such as omega-3 supplementation and the Mediterranean diet have been proposed as complementary approaches to modulate symptoms and improve tolerability. Objectives: To evaluate the efficacy of omega-3 fatty acid supplementation and adherence to the Mediterranean diet on ADHD symptoms in children and adolescents. Methods: A systematic review was conducted in PubMed, Scopus, and Web of Science using the terms “Diet, Mediterranean”, “Fatty Acids, Omega-3”, and “Attention Deficit Disorder with Hyperactivity”. Studies published up to 2026 were included if they examined these interventions in participants under 18 with ADHD. Results: Twenty-three studies involving 1,838 participants met inclusion criteria. Most evaluated omega-3 supplementation, reporting mixed but generally favorable effects on attention, impulsivity, and hyperactivity. In several trials, combining omega-3 with pharmacotherapy reduced stimulant doses and adverse events. Only one randomized trial examined the Mediterranean diet, showing modest improvements limited by low adherence. Conclusion: Omega-3 supplementation may provide modest but meaningful benefits for core ADHD symptoms, particularly when combined with standard treatment. Evidence for the Mediterranean diet remains preliminary and requires further high-quality research.