Precise regulation of the Notch signaling pathway is crucial, as increases or deficiencies in signaling are associated with diseases, including a wide range of cancers. Recent studies have demonstrated that monoclonal antibodies that bind and stabilize the structure of the negative regulatory region (NRR) in the extracellular domain of the Notch receptor can inhibit Notch signaling. In this work, we posed the question whether bicyclic peptides, being around 100-fold smaller than antibodies, can also stabilize the NRR and inhibit Notch signaling. Bicyclic peptides that bind the NRR of human Notch1 were isolated from combinatorial libraries by phage display. Affinity maturation yielded ligands with dissociation constant (K-d) values as low as 150nM. The bicyclic peptides increased the melting temperature of the NRR by up to 8 degrees C, thus substantially stabilizing the protein structure, but they did not inhibit Notch signaling in cellular assays. Although Notch signaling could not be inhibited, this work demonstrates that phage-selected bicyclic peptides can stabilize proteins; this capacity of bicyclic peptides may be exploited for modulating the conformation of other disease targets.