Many children with childhood-onset obsessive-compulsive disorder (OCD) fail to respond adequately to standard therapies. Evidence from preclinical and clinical studies suggests that the glutamatergic neurotransmitter system might be an alternative treatment target. This study examined the efficacy of riluzole, a glutamatergic modulator, as an adjunctive therapy for children with treatment-resistant OCD. In a 12-week, double-blind, placebo-controlled study, 60 treatment-resistant children and adolescents (mean age=14.5 ± 2.4 years), with moderate to severe OCD (mean Children's Yale-Brown Obsessive-Compulsive Scale (CY-BOCS)=28.2 ± 3.7), 17 of whom also had concomitant autism spectrum disorder, were randomized to receive riluzole (final dose of 100 mg/day) or placebo in addition to the existing treatment regimen. Fifty-nine subjects completed the randomized trial. Primary outcome measures were changes on the CY-BOCS, the Clinical Global Impressions Scale, and the Children's Global Assessment Scale. Riluzole was fairly well tolerated, although it was associated with one case of pancreatitis and five instances of slight increases in transaminases. All subjects showed significant reductions in CY-BOCS scores during treatment; however, there was no significant difference between placebo and riluzole on any of the primary or secondary outcome measures. The study failed to demonstrate superiority of riluzole over placebo as an adjunctive treatment for children with childhood-onset OCD. However, future studies may show benefits for less treatment-refractory children with fewer concomitant medications.