In contrast to large proteins, small peptide catalysts typically display limited specificity for small molecule substrates. This is presumably a result of the limited opportunities small peptides have to fold in a manner that provides for the formation of an isolated reaction vessel that effectively binds and sequesters substrates from bulk solvent while at the same time catalyzing their transformation. For the preparation of small peptide catalysts that possess improved substrate specificity, we have developed a modular assembly strategy that involves appending phage display-derived substrate binding-domain modules to catalytically active peptide domains. We demonstrate the potential of this strategy with the construction of a small 35-amino acid residue aldolase peptide with improved substrate specificity. The advantages of this approach are that it reduces the demand on the functionalization of the catalytic site and it is modular, therefore making its adaptation to a variety of specificities rapid. The modular assembly strategy studied here may present advantages over exhaustive searches of large random-sequence peptide libraries for peptides with singular function.