German submarine U-1226 was a Type IXC/40 U-boat of Nazi Germany's Kriegsmarine during World War II.

The U-boat, built for service in the Battle of the Atlantic, was completed in Hamburg in November 1943, and placed under the command of Oberleutnant zur See August-Wilhem Claussen (Crew X/37), whose brother Emil had been killed on board U-469 the previous year.

She underwent working up cruises in the Baltic Sea before embarking on her only operational patrol from Horten Naval Base in Norway during September 1944.

The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 shaft horsepower (1,010 PS; 750 kW) for use while submerged.

It is possible she was sunk in an unrecorded encounter with an Allied ship or aircraft, or more likely she suffered some unknown catastrophic accident which claimed the boat and all its crew.