The interior of the building is even more ornate and luxurious. Within the rotunda, there are 27 varieties of marble, sourced from across the United States, as well as Austria, Belgium, France, Greece, and Italy. The rotunda has two tiers of columns. The first tier, starting from the floor, is composed of 48 light-colored Doric columns, as well as eight darker Ionic columns with gilded tops. These Ionic columns frame four separate wall niches, spaced around the rotunda, that each hold a gilded bronze sculpture by James Earle Fraser.
Above this first tier colonnade is an architrave, a red marble band that connects the columns together. It is broken by the eight arches that frame the eight entrances around the rotunda. Above the architrave is a carved marble frieze, made of a cream-colored marble sourced from Virginia, which is used heavily outside of the rotunda as well. The second tier colonnade is composed of 24 Corinthian columns in darker but varied colors. Half of the spaces between these columns are large art glass windows, while the other half are panels, filled with allegorical mural paintings by Eugene Savage. The ceiling is a gilded dome with five rows of octagonal coffering that get smaller as they approach a central oculus skylight.