Type U 31 was a class of U-boats built during World War I by the Kaiserliche Marine.

Between 1912 and 1915 eleven were built on Germaniawerft in Kiel, amongst these top-three-scoring U-35 with the famous Lothar von Arnauld de la Perière as commander, U-39 with Walter Forstmann and U-38 with Max Valentiner.

[2] Later admiral and head of the Abwehr from 1935 to 1944 Wilhelm Canaris also served as commander on two different Type U 31 U-boats.

[3] Type U-31s were fitted with two Germania 6-cylinder two-stroke diesel engines with a total of 1,850 metric horsepower (1,361 kW; 1,825 bhp) for use on the surface and two Siemens-Schuckert double-acting electric motors with a total of 1,200 PS (883 kW; 1,184 shp) for underwater use.

Cruising range was 8,790 nautical miles (16,280 km; 10,120 mi) at 8 knots (15 km/h; 9.2 mph) on the surface.