L-Carnosine is an amino acid that acts as an anti-oxidant, anti-toxic and neuroprotective agent. There is a paucity of data about the effectiveness of L-Carnosine in the management of autism spectrum disorder (ASD) in children. This study aimed at investigating the effectiveness of L-Carnosine as adjunctive therapy in the management of ASD. This was a randomized controlled trial. Children aged 3-6 years with a diagnosis of mild to moderate ASD were assigned to standard care arm (occupational and speech therapy) and intervention care arm (L-Carnosine, 10-15 mg/kg in 2 divided doses) plus standard care treatment. The children were assessed at the baseline and the end of 2 months for the scores of Childhood Autism Rating Scale, Second Edition-Standard Version (CARS2-ST), Autism Treatment Evaluation Checklist (ATEC), BEARS sleep screening tool and 6-item Gastrointestinal Severity Index (6-GSI). Of the sixty-seven children enrolled, sixty-three children had completed the study. No statistically significant difference (p > 0.05) was observed for any of the outcome measures assessed. Supplementation of L-Carnosine did not improve the total score of CARS2-ST, ATEC, BEARS sleep screening tool and 6-GSI scores of children with ASD. Further investigations are needed with more objective assessments to critically validate the effectiveness of L-Carnosine on ASD children for more decisive results.