German submarine U-138 was a Type IID U-boat of Nazi Germany's Kriegsmarine in World War II.

Her keel was laid down on 16 November 1939 by Deutsche Werke in Kiel as yard number 267.

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 410 metric horsepower (300 kW; 400 shp) for use while submerged.

For her second foray, U-138 sank Bonheur and damaged British Glory on 15 October 1940 northwest of the Butt of Lewis in the Outer Hebrides.

Patrol number three involved another passage west of the British Isles so that the submarine could safely dock in Kiel, but she did not add to her score.

U-138 was attacked by the British destroyers HMS Faulknor, Fearless, Forester, Foresight and Foxhound west of Cadiz, Spain, on 18 June 1941.