German submarine U-42 was a Type IXA[3] U-boat of Nazi Germany's Kriegsmarine that operated during World War II.

Her keel was laid down on 21 December 1937 by DeSchiMAG AG Weser of Bremen as yard number 947.

She was launched on 16 February 1939 and commissioned on 15 July of that same year under the command of Kapitänleutnant Rolf Dau.

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.

[5] During this operation, one enemy ship was damaged, the 4,803 GRT British steam freighter SS Stonepool, which had become separated from Convoy OB 17 while sailing from Liverpool, England to North America.

On 13 Oct U-42 battle surfaced and engaged the ship with her deck gun, hitting her several times before being driven back under by the impressively accurate return fire from the merchantman.

[8] The youngest crew member aboard U-42 was Rudolf Nuggel who was born on 22 December 1919 and was among the 26 dead.