Christian August of Holstein-Gottorp-Eutin was a cadet of the reigning ducal House of Holstein-Gottorp who became prince of Eutin, prince-bishop of Lübeck and regent of the Duchy of Holstein-Gottorp.

Holstein-Gottorp is the historiographical name, as well as contemporary shorthand name, for the parts of the duchies of Schleswig and Holstein, also known as Ducal Holstein, that were ruled by the dukes of Schleswig-Holstein-Gottorp, a side branch of the elder Danish line of the German House of Oldenburg. Other parts of the duchies were ruled by the kings of Denmark.