German submarine U-18 was a Type IIB U-boat of Nazi Germany's Kriegsmarine during World War II.

The submarine was powered by two MWM RS 127 S four-stroke, six-cylinder diesel engines of 700 metric horsepower (510 kW; 690 shp) for cruising, two Siemens-Schuckert PG VV 322/36 double-acting electric motors producing a total of 460 metric horsepower (340 kW; 450 shp) for use while submerged.

[2] While a training boat, U-18 sank at 0954 hrs on 20 November 1936 in Lübeck Bay, after a collision with T-156.

[3] Starting late 1942 she served in the 30th U-boat Flotilla, after being transported in sections along the Danube to the Romanian port of Galați.

[4] On 20 August 1944, in a Soviet air raid on the Romanian harbor of Constanţa in the Black Sea, U-18 was damaged and as a result was deemed not seaworthy and was scuttled on the 25th.