WholeCellKB is a collection of open-access model organism databases specifically designed to enable whole-cell models. Currently, WholeCellKB contains a database of Mycoplasma genitalium, a Gram-positive bacterium and common human pathogen. The M. genitalium database is the most comprehensive description of any single organism to date, and was used to develop the first whole-cell computational model. The M. genitalium database was curated from over 900 primary research articles, reviews, books, and databases. The M. genitalium database is extensively cross-referenced to external resources including BioCyc, KEGG, and UniProt. WholeCellKB is also an open-source web-based software program for constructing model organism databases. The WholeCellKB software provides an extensive and fully customizable data model that fully describes individual species including the structure and function of each gene, protein, reaction, and pathway. WholeCellKB is freely accessible via a web-based user interface as well as via a RESTful web service.