SM UC-20 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) during World War I. The U-boat was ordered on 29 August 1915 and was launched on 1 April 1916. She was commissioned into the German Imperial Navy on 7 September 1916 as SM UC-20. In 13 patrols UC-20 was credited with sinking 21 ships, either by torpedo or by mines laid. UC-20 was surrendered on 16 January 1919 and broken up at Preston in 1919–20.
Like all pre-UC-25 Type UC II submarines, UC-20 had a displacement of 417t when at the surface and 493t while submerged. She had a total length of 49.35m (161.91feet), a beam of 5.22m (17.13feet), and a draught of 3.650NaN0. The submarine was powered by two six-cylinder four-stroke diesel engines each producing 250PS (a total of 500PS), two electric motors producing 460PS, and two propeller shafts. She had a dive time of 35 seconds and was capable of operating at a depth of .
The submarine had a maximum surface speed of and a submerged speed of . When submerged, she could operate for at ; when surfaced, she could travel at . UC-20 was fitted with six mine tubes, eighteen UC 200 mines, three torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 8.8sp=usNaNsp=us Uk L/30 deck gun. Her complement was twenty-six crew members.
| Date | Name | Nationality | Tonnage[1] | Fate[2] | |
|---|---|---|---|---|---|
| 19 October 1916 | Frits Emil |  Denmark | 194 | Sunk | |
| 17 November 1916 | Emilia |  Portugal | 1,159 | Sunk | |
| 10 April 1917 | Abd Razik |  Tunisia | 25 | Sunk | |
| 11 April 1917 | Candia | 1,045 | Sunk | ||
| 14 April 1917 | Cinque Ottobre | 39 | Sunk | ||
| 14 April 1917 | Progresso | 31 | Sunk | ||
| 15 April 1917 | Alessio Cocco | 29 | Sunk | ||
| 18 May 1917 | Millicent Knight |  United Kingdom | 3,563 | Sunk | |
| 25 May 1917 | Argentina | 97 | Sunk | ||
| 25 May 1917 | Ida | 46 | Sunk | ||
| 26 May 1917 | Unione Salvatore | 57 | Sunk | ||
| 26 May 1917 | Abd es Salem |  France | 25 | Sunk | |
| 26 May 1917 | Dandolo | France | 50 | Sunk | |
| 26 May 1917 | Manoubia | France | 50 | Sunk | |
| 26 May 1917 | Messaouda | France | 50 | Sunk | |
| 26 May 1917 | San Francesco | 47 | Sunk | ||
| 27 May 1917 | Boldwell | United Kingdom | 3,118 | Sunk | |
| 26 August 1917 | Maurizio P. | 558 | Sunk | ||
| 4 January 1918 | Regina Elena | 7,940 | Sunk | ||
| 13 April 1918 | Giove | 5,037 | Damaged | ||
| 28 April 1918 | Verdun | France | 2,769 | Sunk | |
| 4 May 1918 | Mergellina | 354 | Damaged | ||
| 4 June 1918 | Strombus | United Kingdom | 6,163 | Damaged | |
| 12 June 1918 | Poincare | Tunisia | 35 | Sunk |