Sleep disorders are frequently reported in autistic patients. Evidences suggest that increased oxidative stress and reduced antioxidants may play a major role in the pathogenesis of these disorders. Carnosine acts as an antioxidant, antitoxic and neuroprotective agent. The aim of this trial study was to examine the effects of carnosine supplementation on the sleep disorders and severity of autism core symptoms in autistic patients. In this double-blind, randomized clinical trial, 43 autistic patients (31 boys and 12 girls; aged 4 to 16 years) were divided into two groups of carnosine and control that received 500 mg of carnosine and 500 mg of placebo per day for 2 months, respectively. Sleep disorders were measured using Children's Sleep Habits Questionnaires. Gilliam Autism Rating Scale 2 was used to assess the effects of carnosine supplementation on the autism severity. Carnosine supplementation did not change anthropometric indices (p > 0.05) and showed no effect on autism severity (p > 0.05), whereas it significantly reduced sleep duration (p = 0.04), parasomnias (p = 0.02) and total sleep disorders score by 7.59% (p = 0.006) when compared with the control group. The results suggest that carnosine supplementation could be effective in improving sleep disturbances, in particular sleep duration and parasomnias subscales.