This study identifies a thermal maturity anomaly within the downdip Mississippi Interior Salt Basin (MISB) of southern Mississippi, USA, through examination of bitumen reflectance data from Aptian-age strata (Sligo Formation, Pine Island Shale, James Limestone, and Rodessa Formation). U.S. Geological Survey (USGS) reconnaissance investigations conducted in 2011-2012 examined Aptian-age thermal maturity trends across the onshore northern Gulf of Mexico region and indicated that the section in the downdip MISB is approaching the wet gas/condensate window (R-o similar to 1.2%). A focused study in 2012-2013 used 6 whole core, one sidewall core, and 49 high-graded cutting samples (depth range of 13,000-16,500 ft [3962.4-5029.2 m] below surface) collected from 15 downdip MISS wells for mineralogy, fluid inclusion, organic geochemistry, and organic petrographic analysis. Based on native solid bitumen reflectance (R-o generally >0.8%; interpreted to be post-oil indigenous bitumens matured in situ), R-o values increase regionally across the MISB from the southeast to the northwest. Thermal maturity in the eastern half of the basin (R-o range 1.0 to 125%) appears to be related to present-day burial depth and shows a gradual increase with respect to depth. To the west, thermal maturity continues to increase even as the Aptian section shallows structurally on the Adams County High (R-o range 1.4 to >1.8%). After evaluating the possible thermal agents responsible for increasing maturity at shallower depths (i.e., igneous activity, proximity to salt, variations in regional heat flux, and uplift), we tentatively propose that either greater paleoheat flow or deeper burial coupled with uplift in the western part of the MISB could be responsible for the thermal maturity anomaly. Further research and additional data are needed to determine the cause(s) of the thermal anomaly. Published by Elsevier B.V.