The term Reichskriegsflagge (German: [ˈʁaɪçsˌkʁiːksflaɡə], lit. 'Imperial War Flag') refers to several war flags and war ensigns used by the German armed forces in history. A total of eight different designs were used in 1848–1849 and between 1867–1871 and 1945.

Today the term refers usually to the flag from 1867–1871 to 1918, the war flag of Imperial Germany.

After Prussia defeated Austria in 1866, the North German Confederation was founded by Prussia in order to replace the former German Confederation in which Austria had been the dominant power. The new Confederation eventually became the German Empire after the Franco-Prussian War, hence the Imperial German flags date back to the North German Confederation.

Because Prussia had emerged as the leading German state and the black-red-golden colour scheme was, at that time, associated with pan-Germanism (a Germany including Austria) and the revolution of 1848, the Prussian king and President of the North German Confederation, Wilhelm I, insisted on using new flags inspired by Prussia and without the traditional German colours. Therefore, the North German and eventually Imperial German flags prominently featured the Prussian colours (black and white) as well as symbols like the Prussian eagle and the Iron Cross. And while seafaring was the traditional domain of the Hanse in Germany, virtually all of the 19th century German coastline (including the North Sea coast) and naval power belonged to Prussia after the decisive victory in 1866.

Prince Adalbert of Prussia, the commander-in-chief of the North German Navy, was commissioned with the task of designing the new war ensign. Around 1850, he already had designed a number of potential war ensigns for a hypothetical German nation-state under Prussian rule, which never were adopted as the project was not realised at that time. He used his old ideas to design the new North German war ensign with heavy involvement of the Prussian king, leading to numerous changes, like the inclusion of the Iron Cross in its original shape.

The flag that was finally adopted became the war ensign of the German Empire after its founding, similar to the North German merchant ensign.

The first German imperial war flag was introduced by imperial law on 12 November 1848. The Regent signed it that day, although the National Assembly had already voted for it on 31 July. The law described the commercial and war flag of the emerging new German federal state. The 'German colours' – black, red and gold – were so popular from their beginnings on a Thuringian principality's state flag in 1778 that it did not seem necessary to mention them in the constitution of 1849. The flag was used for the Imperial Fleet. The short battle near Heligoland on 4 June 1849 was the first and only maritime battle in which these colours were involved.

The Federal Diet had already adopted the German colours on 9 March 1848, but the Diet and also the Central Powers forgot to announce the new flag to the foreign powers. By May 1850, the flag was recognised by the U.S., the Netherlands, Belgium, Sardinia, the Ottoman Empire, Portugal, the Kingdom of the Two Sicilies, Spain, Greece, and (conditionally) France. The reestablished German Confederation took over the fleet, but sold it in 1852 and did not make use of the German colours again until the 1860s.