German submarine U-3509 was a Type XXI U-boat of Nazi Germany's Kriegsmarine during World War II.

The Elektroboote submarine was laid down on 29 July 1944 at the Schichau-Werke yard at Danzig, launched on 27 September 1944, and commissioned on 29 January 1945 under the command of Oberleutnant zur See Karl-Heinz Voswinkel.

U-3509 was a brand new, high technology electric boat which could run constantly submerged rather than having to surface to recharge her batteries every day the way submarines until that point had had to do.

[5] The submarine was powered by two MAN SE supercharged six-cylinder M6V40/46KBB diesel engines each providing 4,000 metric horsepower (2,900 kilowatts; 3,900 shaft horsepower), two Siemens-Schuckert GU365/30 double-acting electric motors each providing 5,000 PS (3,700 kW; 4,900 shp), and two Siemens-Schuckert silent running GV232/28 electric motors each providing 226 PS (166 kW; 223 shp).

[5] U-3509 was damaged whilst under construction in September 1944 during an air raid, but she was eventually repaired and completed.