Current methods of structure identification in mass spectrometry based non-targeted metabolomics rely on matching experimentally determined features of an unknown compound to those of candidate compounds contained in biochemical databases. A major limitation of this approach is the relatively small number of compounds currently included in these databases. If the correct structure is not present in a database it cannot be identified, and if it cannot be identified it cannot be included in a database. Thus, there is an urgent need to augment metabolomics databases with rationally designed biochemical structures using alternative means. In this study, we present a database of in silico enzymatically synthesized metabolites (IIMDB) to partially address this problem. The database, which is available from http://metabolomics.pharm.uconn.edu/iimdb/, includes ~23,000 known compounds (mammalian metabolites, drugs, secondary plant metabolites and glycerophospholipids) collected from existing biochemical databases plus more than 400,000 computationally generated human phase I and phase II metabolites of these known compounds. The IIMDB database features a user-friendly web interface and a programmer-friendly RESTful web service. Ninety-five percent of the computationally generated metabolites in IIMDB were not found in any existing database. However, 21,640 were identical to compounds already listed in PubChem, HMDB, KEGG or HumanCyc. Furthermore, a vast majority of these in silico metabolites were scored as biological using BioSM, a software program that identifies biochemical structures in chemical structure space. These results suggest that in silico biochemical synthesis represents a viable approach for significantly augmenting biochemical databases for non-targeted metabolomics applications.