Lactate dehydrogenase 5 (LDH5) is overexpressed in aggressive and metastatic tumors and is an attractive target for anticancer therapy. Small molecule drugs have been developed to target the substrate and/or the cofactor sites of LDH5, but none has reached the clinic to date, and alternative strategies remain almost unexplored. Combining rational and computer-based approaches, we identified peptidic sequences with high affinity towards a β-sheet region that is involved in protein-protein interactions (PPIs) required for the activity of LDH5. To improve structural stability and potency, these sequences were grafted into a cyclic cell penetrating β-hairpin peptide scaffold. The lead grafted peptide, cGmC9, inhibited LDH5 activity in vitro in low micromolar range and more efficiently than the small molecule inhibitor GNE-140. cGmC9 inhibits LDH5 by targeting an interface unlikely to be inhibited by small molecule drugs. This successful lead will guide the development of new LDH5 inhibitors and challenges the landscape of drug discovery programs exclusively dedicated to small molecules.