The role of cholinergic abnormalities in autism was recently evidenced and there is a growing interest in cholinergic modulation, emerging for targeting autistic symptoms. Galantamine is an acetylcholinesterase inhibitor and an allosteric potentiator of nicotinic receptors. This study aimed to evaluate the possible effects of galantamine as an augmentative therapy to risperidone, in autistic children. In this randomized, double-blind, placebo-controlled, parallel-group study, 40 outpatients aged 4–12 years whom had a diagnosis of autism (DSM IV-TR) and a score of 12 or higher on the Aberrant Behavior Checklist-Community (ABC-C) Irritability subscale were equally randomized to receive either galantamine (up to 24 mg/day) or placebo, in addition to risperidone (up to 2 mg/day), for 10 weeks. We rated participants by ABC-C and a side effects checklist, at baseline and at weeks 5 and 10. By the study endpoint, the galantamine-treated patients showed significantly greater improvement in the Irritability (P = 0.017) and Lethargy/Social Withdrawal (P = 0.005) subscales than the placebo group. The difference between the two groups in the frequency of side effects was not significant. In conclusion, galantamine augmentation was shown to be a relatively effective and safe augmentative strategy for alleviating some of the autism-related symptoms.