U-139, originally designated "Project 46", was a class of large, long-range U-boats built during World War I by the Kaiserliche Marine.

Displacing nearly 2,000 tons, and with a surface speed of 15 knots (28 km/h; 17 mph), they were armed with 24 torpedoes and two 15 cm deck guns, and had a cruising range of around 17,000 nautical miles (31,000 km; 20,000 mi).

Four bow and two stern torpedo tubes were fitted, but the main armament was the two 15 cm deck guns, which could be laid on target by a rangefinder on the aft section of the bridge.

[1] An even larger U-cruiser was proposed under "Project 47", but never reached construction; it would have displaced 2,500 tons, had a top speed of 21 knots (39 km/h; 24 mph), and been armed with four 150 mm guns as well as six torpedo tubes, two of which would have fired to the side.

[citation needed] Lothar von Arnauld de la Perière commanded U-139, the first of the class, and named the submarine Kapitänleutnant Schweiger, after Walther Schwieger, who had sunk RMS Lusitania in 1915.