SM U-20 or U-XX was the lead boat of the U-20 class of submarines or U-boats built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

When it became apparent to the Austro-Hungarian Navy that the First World War would not be a short war,[1] they moved to bolster their U-boat fleet by seizing the plans for Denmark's Havmanden class submarines,[7] which had been designed by Whitehead & Co. in Fiume, who had built three units.

[7] Due to demands by the Hungarian government,[1] subcontracts for the class were divided between Hungarian and Austrian firms,[2] but this politically expedient solution worsened technical problems with the design and resulted in numerous modifications and delays for the class in general.

The collision with the Austro-Hungarian light cruiser twisted U-20's periscope, extensively damaged the conning tower, and knocked off her deck gun.

Italian salvage crews raised the boat's rear portion on 22 July and her front section on 21 November.

[4] The conning tower and a small midships section of U-20 were donated to the Heeresgeschichtliches Museum in Vienna, where they are on display, while the rest of the boat was scrapped.

[7] The remains of the crewmen were buried on the grounds of the Theresian Military Academy at Wiener Neustadt.