 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. Kriegsmarine) during the First World War. The design for U-20 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.
