Hagen im Bremischen belonged to the Prince-Archbishopric of Bremen, established as a territory of imperial immediacy in 1180.

The prince-archiepiscopal fortress (Latin: Castrum Hagen, German: Burghagen) dates back to the 12th century, probably Prince-Archbishop Hartwig II initiated its construction.

In 1648 the Prince-Archbishopric was transformed into the Duchy of Bremen, which was first ruled in personal union by the Swedish crown – interrupted by a Danish occupation (1712–1715) – and from 1715 on by the House of Hanover.

The oak bears the coat of arms of the Prince-Archbishopric of Bremen, which refers to the second name element im Bremischen, meaning literally in the Bremian (country).

The Bremian coat of arms displays two argent (silver) crisscrossed keys on a gules (red) background.