In mass spectrometry-based proteomics, most conventional search engines match spectral data to sequence databases. These search databases thus play a crucial role in the identification process. While search engines can derive peptides in silico from protein sequences, this is usually limited to standard digestion algorithms. Customized search databases that provide detailed control over the search space can vastly outperform such standard strategies, especially in gel-free proteomics experiments. Here we present Database on Demand, an easy-to-use web tool that can quickly produce a wide variety of customized search databases.