SM U-21 or U-XXI was a U-20-class submarine or U-boat built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

The design for U-21 was based on submarines of the Royal Danish Navy's Havmanden class (three of which had been built in Austria-Hungary), and was largely obsolete by the beginning of the war.

The U-boat conducted patrols off the Albanian coast in October 1917, but experienced the failure of the seal on her main hatch.

When it became apparent to the Austro-Hungarian Navy that the First World War would not be a short one,[1] they moved to bolster their U-boat fleet by seizing the plans for the Danish Havmanden class submarines,[6] three of which had been built at Whitehead & Co. in Fiume.

On 29 September, von Seyffertitz steered the boat from the submarine base at Brioni to Cattaro, where she arrived on 1 October.

Slated to sail into the Ionian Sea, U-21 instead had to turn back the following day when the main hatch seal on the conning tower leaked and could not be repaired.

[10] Dürrial led U-21 out of Pola on 1 June for Cattaro, making stops en route at Arbe and Novigrad for repairs to the gyrocompass.

[2] On 16 July, while conducting patrols off the Albanian coast, a piston in U-21's diesel engine broke and Dürrial put in at Djenovic.