SM U-23 or U-XXIII was a U-20-class submarine or U-boat built for and operated by the Austro-Hungarian Navy () during the First World War. The design for U-23 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. U-23 was just over long and was armed with two bow torpedo tubes, a deck gun, and a machine gun. In February 1918, U-23 was sunk with all hands by the Italian destroyer Airone while attempting an attack on the Italian transport Memfi. U-23 had no wartime successes.