Dietary interventions such as restrictive diets or supplements are common treatments for young people with autism spectrum disorder (ASD). Evidence for the efficacy of these interventions is still controversial. To assess the efficacy of specific dietary interventions on symptoms, functions, and clinical domains in subjects with ASD by using a meta-analytic approach. Ovid Medline, PsycINFO, Embase databases. We selected placebo-controlled, double-blind, randomized clinical trials assessing the efficacy of dietary interventions in ASD published from database inception through September 2017. Outcome variables were subsumed under 4 clinical domains and 17 symptoms and/or functions groups. Hedges' adjusted g values were used as estimates of the effect size of each dietary intervention relative to placebo. In this meta-analysis, we examined 27 double-blind, randomized clinical trials, including 1028 patients with ASD: 542 in the intervention arms and 486 in the placebo arms. Participant-weighted average age was 7.1 years. Participant-weighted average intervention duration was 10.6 weeks. Dietary supplementation (including omega-3, vitamin supplementation, and/or other supplementation), omega-3 supplementation, and vitamin supplementation were more efficacious than the placebo at improving several symptoms, functions, and clinical domains. Effect sizes were small (mean Hedges' g for significant analyses was 0.31), with low statistical heterogeneity and low risk of publication bias. Methodologic heterogeneity among the studies in terms of the intervention, clinical measures and outcomes, and sample characteristics. This meta-analysis does not support nonspecific dietary interventions as treatment of ASD but suggests a potential role for some specific dietary interventions in the management of some symptoms, functions, and clinical domains in patients with ASD.