The Geochemical Database for Iron Oxide-Copper-Cobalt-Gold-Rare Earth Element Deposits of Southeast Missouri (IOCG-REE_GX) contains new geochemical data compilations for samples from IOCG-REE type deposits in which each rock sample has one "best value" determination for each analyzed species, greatly improving speed and efficiency of use. IOCG-REE_GX was created and designed to compile whole-rock and trace element data from southeast Missouri in order to facilitate petrologic studies, mineral resource assessments, and the definition and statistics of geochemical baseline values. This relational database serves as a data archive in support of present and future geologic and geochemical studies of IOCG-REE type deposits, and contains data tables in two different formats describing historical and new quantitative and qualitative geochemical analyses. The analytical results were determined by 24 laboratory analytical methods on 457 rock samples collected during two phases from outcrop and drill core sites from throughout the entire St. Francois Mountains terrane made by the U.S. Geological Survey (USGS). In the first phase, the USGS collected and analyzed 315 samples from 1989 to 1995. During the second phase from 2013-2015, 119 samples were collected and analyzed, and 23 samples from the first phase were reanalyzed using analytical methods of higher precision. The data presents the most precise analytical approach to report the best value for each element. In order to facilitate examination of the geochemistry of the broad range of samples reported (i.e., regional samples, ore zone, or ore deposit alteration-related), a short sample description is given and each sample is coded (Jcode) according to the type of rock suites defined by Kisvarsanyi (1981), whether the sample was collected as a non-ore deposit-related representative of a given rock suite or as a deposit-related sample (Kcode, and; if the sample was related to a specific ore deposit the zone within the given deposit was included (Lcode. These coded data provide a robust tool for evaluating the regional geologic setting of the host terrane as well as assessing the character of hydrothermal alteration related to many of the contained mineral deposits. Data from the first phase are currently maintained in the USGS National Geochemical Database (NGDB), and data from the second phase will soon be added. The data of the IOCG-REE_GX were checked for accuracy regarding sample location, sample media type, and analytical methods used.