SM U-23[Note 1] was one of the 329 U-boats serving in the Imperial German Navy in World War I. U-23 was engaged in the naval warfare and took part in the First Battle of the Atlantic.

U-23 served on three war patrols, sinking a total of seven ships for 8,822 gross register tons (GRT).

She was baited by the Q ship Princess Louise and torpedoed by HMS C27 at 58°55′N 0°14′E / 58.917°N 0.233°E / 58.917; 0.233, off Fair Isle, in Shetland, Scotland.

Twenty four men died and 10 survived.