Premature termination codons in mRNAs result from nonsense mutations and hinder the translation of full-length, functional proteins. Nonsense mutations cause numerous serious genetic diseases, including cystic fibrosis and Duchenne muscular dystrophy. Several small-molecule drugs have been reported that could potentially ameliorate these diseases by promoting translational readthrough at the premature termination codon. However, utilization of many of these molecules faces problems such as limited efficacy or high cellular toxicity. Using a selection strategy in Saccharomyces cerevisiae coupling suppression of endogenous nonsense mutations to cell survival, we identified ten readthrough-promoting cyclic peptides from a DNA-encoded library. The selected cyclic peptides suppress nonsense mutations in various reporter genes, and the candidates inducing the highest readthrough levels display no observable cytotoxicity in yeast. Mutational analysis of the most promising cyclic peptide demonstrate that most amino acid side chains contribute to the readthrough-stimulating activity. Importantly, this cyclic peptide appears to bind directly to the eukaryotic core translation machinery and promotes readthrough in vitro by interfering with ribosomal decoding. Our results suggest that small, cyclic peptides selected in vivo could represent a novel drug type to treat the many incurable human genetic diseases that are caused by nonsense mutations.