The I Army Corps / I AK (German: I. Armee-Korps) was a corps level command of the Prussian and then the Imperial German Armies from the 19th Century to World War I.

It was established with headquarters in Königsberg (now Kaliningrad, Russia). Initially, the Corps catchment area comprised the entire Province of East Prussia, but from 1 October 1912 the southern part of the Province was transferred to the newly formed XX Corps District.

In peacetime, the Corps was assigned to the I Army Inspectorate, which became the 8th Army at the start of the First World War. The corps was still in existence at the end of the war, and was disbanded with the demobilisation of the German Army after World War I.

The I Corps fought in the Austro-Prussian War against Austria in 1866, including the Battle of Trautenau and the Battle of Königgrätz.

The Corps served in the Franco-Prussian War against France in 1870–1871. It saw action in the Battle of Noiseville, the Battle of Gravelotte, the Siege of Metz, the Battle of Amiens, the Battle of Hallue, and the Battle of St. Quentin, among other actions.

From formation in 1820, the Corps commanded two divisions: 1st Division and 2nd Division. These were joined by 37th Division when it was formed on 1 April 1899. 37th Division was transferred to XX Corps when it was formed on 1 October 1912.

The 25 peacetime Corps of the German Army (Guards, I - XXI, I - III Bavarian) had a reasonably standardised organisation. Each consisted of two divisions with usually two infantry brigades, one field artillery brigade and a cavalry brigade each. Each brigade normally consisted of two regiments of the appropriate type, so each Corps normally commanded 8 infantry, 4 field artillery and 4 cavalry regiments. There were exceptions to this rule:

Each Corps also directly controlled a number of other units. This could include one or more