German submarine U-20 was a Type IIB U-boat of Nazi Germany's Kriegsmarine. Her keel was laid down on 1 August 1935, by Germaniawerft of Kiel as yard number 550. She was commissioned on 1 February 1936. During World War II, she conducted operations against enemy shipping.

The Type II U-boat was designed by Nazi Germany as a coastal U-boat, modeled after the CV-707 submarine, which was designed by the Dutch dummy company NV Ingenieurskantoor voor Scheepsbouw Den Haag (I.v.S) and built in 1933 by the Finnish Crichton-Vulcan shipyard in Turku, Finland. It was too small to undertake sustained operations far away from the home support facilities. Its primary role was found to be in the training schools, preparing new German naval officers for command. It appeared in four sub-types.