The IX Army Corps / IX AK () was a corps level command of the Prussian and German Armies before and during World War I. IX Corps was one of three formed in the aftermath of the Austro-Prussian War (the others being X Corps and XI Corps). The Corps was formed in October 1866 with headquarters in Altona. The catchment area included the newly annexed Province of Schleswig-Holstein, the Grand Duchies of Mecklenburg-Schwerin and Mecklenburg-Strelitz and the Hanseatic cities of Lübeck, Hamburg and Bremen. During the Franco-Prussian War it was assigned to the 2nd Army.