The instant disclosure features single-stranded RNA molecules comprising one or more internal, non-nucleotide spacers. A non-nucleotide spacer covalently links nucleotide portions of the molecule. The single-stranded RNA molecules function as guide or antisense strands that are capable of inhibiting gene expression via an RNA interference mechanism and, thus, represent single-stranded RNAi agents. The single-stranded RNAi molecules can be used in methods for a variety of therapeutic, diagnostic, target validation, genomic discovery, genetic engineering, and pharmacogenomic applications.