The instant disclosure features, among other things, compositions and methods for treating an autism spectrum disorder in a human. The compositions comprise an effective amount of: (1) an isothiocyanate (e.g., sulforaphane or a derivative thereof) or (2) a glucosinolate, and optionally, an enzyme, to thereby treat an autism spectrum disorder and/or reduce the severity of at least one symptom of the disorder. Methods for preparing such compositions are also featured.