The district as known today was created in 1969, during the reorganization of the districts in North Rhine-Westphalia, by merging Sieg District with the District of Bonn (from which Bonn itself was separated in 1887 to become an urban district). Sieg District was created in 1825.
Geographically Rhein-Sieg District covers the valley of the river Sieg and also, since the merger with the District of Bonn, that of the Rhine around Bonn, as well an area in the most easterly part of the Eifel.
The red lion is the symbol of the dukes of Berg, who owned a large part of the district. The black cross in the shield is the symbol of Cologne, because another part of the district was owned by the bishops of Cologne. The yellow sword is the symbol of Saint Michael, the patron of the mighty Michaelsberg Abbey in Siegburg.