SM U-121 was a Type UE II long-range minelaying U-boat of the Imperial German Navy intended for service in the Mediterranean. The Austro-Hungarian Navy allocated her the number SM U-84. She was built at Hamburg, Germany, by Aktiengesellschaft Vulcan and launched on 20 September 1918. Completed after the Armistice she was never commissioned in the Imperial German Navy but handed over to France on 9 March 1919. She was sunk as target off Cherbourg on 1 July 1921.
|Builder:||AG Vulcan, Hamburg|
|Launched:||20 September 1918|
|In service:||9 March 1919|
|Class and type:||German Type UE II submarine|
|Type:||Coastal minelaying submarine|
|Length:||81.52 m (267 ft 5 in) (o/a)|
|Beam:||7.42 m (24 ft 4 in)|
|Height:||10.16 m (33 ft 4 in)|
|Draught:||4.22 m (13 ft 10 in)|
|Propulsion:||2 shafts, 2 × 1.61 m (5 ft 3 in) propellers|
|Test depth:||75 m (246 ft)|
|Complement:||4 officers, 36 enlisted|
- "SM" stands for "Seiner Majestät" (English: His Majesty's) and combined with the U for Unterseeboot would be translated as His Majesty's Submarine.
- Gröner 1991, p. 15.
- Gröner, Erich; Jung, Dieter; Maass, Martin (1991). U-boats and Mine Warfare Vessels. German Warships 1815–1945. 2. Translated by Thomas, Keith; Magowan, Rachel. London: Conway Maritime Press. ISBN 0-85177-593-4.