German submarine U-3040 was a Type XXI U-boat (one of the "Elektroboote") of Nazi Germany's Kriegsmarine, built for service in World War II. She was ordered on 6 November 1943, and was laid down on 9 December 1944 at AG Weser, Bremen, as yard number 1199. She was launched on 10 February 1945, and commissioned under the command of Oberleutnant zur See Heinz Robbert on 8 March 1945.
Like all Type XXI U-boats, U-3040 had a displacement of when at the surface and while submerged. She had a total length of (o/a), a beam of , and a draught of . The submarine was powered by two MAN SE supercharged six-cylinder M6V40/46KBB diesel engines each providing , two Siemens-Schuckert GU365/30 double-acting electric motors each providing , and two Siemens-Schuckert silent running GV232/28 electric motors each providing .
The submarine had a maximum surface speed of and a submerged speed of . When running on silent motors the boat could operate at a speed of . When submerged, the boat could operate at for ; when surfaced, she could travel at . U-3040 was fitted with six torpedo tubes in the bow and four C/30 anti-aircraft guns. She could carry twenty-three torpedoes or seventeen torpedoes and twelve mines. The complement was five officers and fifty-two men.
U-3040 was scuttled on 3 May 1945, at Kiel, as part of Operation Regenbogen. The wreck was later raised and broken up.