German submarine U-34 was a Type VIIA U-boat of Nazi Germany's Kriegsmarine during World War II.

During World War II the boat carried out seven patrols, sinking 22 ships and capturing two more.

The submarine was powered by two MAN M 6 V 40/46 four-stroke, six-cylinder diesel engines producing a total of 2,100 to 2,310 metric horsepower (1,540 to 1,700 kW; 2,070 to 2,280 shp) for use while surfaced, two BBC GG UB 720/8 double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged.

Harald Grosse, she sank the Spanish Republican Navy submarine C-3 off the coast of Malaga on 12 December 1936.

On 7 September she sank Pukkastan about 39 nmi (72 km; 45 mi) southwest of Bishop Rock after getting the ship to stop with two rounds fired across her bows with the deck gun.

The next day she repeated the exercise and sank Kennebec about 70 nmi (130 km; 81 mi) southwest of the Scilly Isles.

U-34's second foray was even more fruitful, sinking Gustav Adolf and Sea Venture (which had replied to the U-boats' warning shots with fire of her own), both on 20 October 1939.

U-34 torpedoed the already scuttled Norwegian minelayer HNoMS Frøya on 13 April 1940 near Søtvika to prevent her salvage.

On 5 July she sank the British destroyer HMS Whirlwind 120 nmi (220 km; 140 mi) west of Lands End.

The boat docked at the newly occupied port of Lorient, on the French Atlantic coast, on 18 July.

She was sunk at 21:55 on 5 August 1943 at Memel (today's Klaipėda in Lithuania), in the Baltic, in position 55°42′N 21°09′E / 55.700°N 21.150°E / 55.700; 21.150 after a collision with the U-boat tender Lech.