SM U-23 or U-XXIII 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.

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,[2] which had been designed by Whitehead & Co. in Fiume, who had built three units.

[4] Although there are no specific reports of problems with U-23, the U-20 class as a whole suffered from unreliable engines which compounded the poor handling characteristics of the boats.

[4] On 21 February 1918,[8] Linienschiffsleutnant Klemens Ritter von Bezard, U-23's only commanding officer,[5] was guiding the boat in an attack on the Italian transport Memfi in the Straits of Otranto.

U-23 came under attack by the Italian torpedo boat Airone which first tried to ram the U-boat, and then deployed an explosive paravane.