The preparation of synthetic molecules showing the remarkable efficiencies characteristic of natural biopolymer catalysts remains a formidable challenge for chemical biology. Although significant advances have been made in the understanding of protein structure and function, the de novo construction of such systems remains elusive. Re-engineered natural enzymes and catalytic antibodies, possessing tailored binding pockets with appropriately positioned functional groups, have been successful in catalysing a number of chemical transformations, sometimes with impressive efficiencies. But efforts to produce wholly synthetic catalytic peptides have typically resulted in compounds with questionable structural stability, let alone reactivity. Here we describe a 33-residue synthetic peptide, based on the coiled-coil structural motif, which efficiently catalyses the condensation of two shorter peptide fragments with high sequence- and diastereoselectivity. Depending on the substrates used, we observe rate enhancements of tenfold to 4,100-fold over the background, with catalytic efficiencies in excess of 10(4). These results augur well for the rational design of functional peptides.