SM U-6 or U-VI was a U-5-class submarine or U-boat built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

The boat was commissioned into the Austro-Hungarian Navy in July 1910, and served as a training boat—sometimes making as many as ten cruises a month—through the beginning of the First World War in 1914.

U-6 was built as part of a plan by the Austro-Hungarian Navy to competitively evaluate foreign submarine designs from Simon Lake, Germaniawerft, and John Philip Holland.

[5] For surface running, U-6 was outfitted with 2 gasoline engines, but suffered from inadequate ventilation, which resulted in frequent intoxication of the crew;[6] her underwater propulsion was by two electric motors.

[11] Linienschiffsleutnant Hugo von Falkhausen, U-6's commander since November 1915,[4] attempted to pass underneath two drifters that formed a part of the Otranto Barrage.

While submerged, von Falkhausen heard an unexplained noise on the hull of the boat, which was likely the sound of U-6 fouling one of the anti-submarine nets deployed from the drifter Calistoga.

Calistoga launched signal flares that attracted the attention of two nearby drifters Dulcie Doris and Evening Star II.

Realizing that he was stuck, and with Dulcie Doris and Evening Star II beginning to shell his boat, U-6's captain ordered code books and confidential material thrown overboard and the submarine scuttled.