An inventory of the vascular flora of the Tennessee River Gorge (TRG) was conducted between April 2009 and July 2011. The TRG is a 41-km-long river canyon located on the Cumberland Plateau physiographic province within Hamilton and Marion counties, TN. According to prior ecological surveys, the 4970-ha study area contains 12 natural communities. This survey documented 123 families, 392 genera, and 700 species and sub-specific taxa, including 138 county range extensions. Approximately 13% of the flora (92 species) is considered non-native. Fifteen imperiled species were reported, including 3 that are undescribed and 9 not documented in any previous Cumberland Plateau flora. In addition to the floristic component of this work, a phytogeographical analysis was conducted for the TRG and 12 other Cumberland Plateau floras in Tennessee and Kentucky. An important finding of this research was the identification of a biogeographical transition zone, from a higher northern to a higher southern species richness. This biogeographic break was identified in the vicinity of Fentress, Scott, and Morgan counties, TN.