jats:pAberrant Ras signalling drives 30% of cancers and inhibition of Rho family small-GTPase signalling has been shown to combat Ras-driven cancers. Here we present the discovery of a 16mer cyclic peptide that binds to Cdc42 with nanomolar affinity. Affinity maturation of this sequence has produced a panel of derived candidates with increased affinity and modulated specificity for other closely related small-GTPases. The structure of the tightest binding peptide was solved by NMR and its binding site on Cdc42 determined. Addition of a cell penetrating sequence allowed the peptides to access the cell interior and engage with their target(s), modulating signalling pathways. In Ras-driven cancer cell models, the peptides have an inhibitory effect on proliferation and show suppression of both invasion and motility. As such they represent promising candidates for Rho-family small GTPase inhibitors and therapeutics targeting Ras-driven cancers. Our data adds to the growing literature demonstrating that peptides are establishing their place in the biologics arm of drug discovery.</jats:p>