Introduction: Attention Deficit Hyperactivity Disorder (ADHD) is a common neurodevelopmental disorder in children and adolescents. Recent research has suggested a potential link between gut microbiota-derived metabolites, such as Trimethylamine N-oxide (TMAO), and neuropsychiatric disorders. This study aims to investigate plasma TMAO levels in children and adolescents with ADHD and explore the relationship between TMAO levels and the severity of ADHD symptoms. Methods: This cross-sectional study included 96 participants aged 7 to 15 years, consisting of 50 patients diagnosed with ADHD and 46 healthy controls. Plasma TMAO levels were measured using the Enzyme-Linked Immunosorbent Assay (ELISA) method. ADHD symptom severity was assessed using the T-DSM-IV-TR scale. Results: Plasma TMAO levels were significantly higher in the ADHD group compared to the control group (p=0.02). Additionally, a positive correlation was found between TMAO levels and the total T-DSM-IV-TR scores (p<0.001), indicating that higher TMAO levels are associated with more severe ADHD symptoms. No significant correlations were observed between TMAO levels and other subscale scores. Conclusion: The findings suggest that elevated plasma TMAO levels in children and adolescents with ADHD may be indicative of the disorder’s biochemical characteristics. Furthermore, TMAO could be linked to the severity of ADHD symptoms, highlighting its potential role in the pathophysiology of ADHD.