German submarine U-40 was a Type IXA[3] U-boat of Nazi Germany's Kriegsmarine that operated during World War II.

U-40 was launched on 9 November 1938 and commissioned on 11 February 1939 under the command of Kapitänleutnant Werner von Schmidt.

The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 metric horsepower (740 kW; 990 shp) for use while submerged.

[4] After being commissioned and deployed, U-40 was stationed in the German port city of Wilhelmshaven,[3] which to be her home for the rest of her fairly short service life.

Choosing to make the voyage nearly three and a half hours after high tide, the mines were not at their lowest point.

Nearly ten hours after the sinking, the remaining three men were rescued and taken prisoner by HMS Boreas.