German submarine U-1223 was a Type IXC/40 U-boat built for Nazi Germany's Kriegsmarine during World War II.

German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-1223 had a displacement of 1,144 tonnes (1,126 long tons) when at the surface and 1,257 tonnes (1,237 long tons) while submerged. The U-boat had a total length of 76.76 m (251 ft 10 in), a pressure hull length of 58.75 m (192 ft 9 in), a beam of 6.86 m (22 ft 6 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.67 m (15 ft 4 in). 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. She had two shafts and two 1.92 m (6 ft) propellers. The boat was capable of operating at depths of up to 230 metres (750 ft).

The submarine had a maximum surface speed of 18.3 knots (33.9 km/h; 21.1 mph) and a maximum submerged speed of 7.3 knots (13.5 km/h; 8.4 mph). When submerged, the boat could operate for 63 nautical miles (117 km; 72 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 13,850 nautical miles (25,650 km; 15,940 mi) at 10 knots (19 km/h; 12 mph). U-1223 was fitted with six 53.3 cm (21 in) torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one 10.5 cm (4.13 in) SK C/32 naval gun, 180 rounds, and a 3.7 cm (1.5 in) Flak M42 as well as two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of forty-eight.

U-1223 was ordered on 25 August 1941 from Deutsche Werft in Hamburg-Finkenwerder under the yard number 386. Her keel was laid down on 25 November 1942 and was launched the following year on 23 June 1943. About three months later she was commissioned into service under the command of Kapitänleutnant Harald Bosüner (Crew 35) on 6 October 1943 in the 4th U-boat Flotilla.

While working up for deployment, U-1223 Bosüner was relieved and handed over command to Oberleutnant zur See Albert Kneip (Crew X/39) in March 1944. After completing training, the U-boat transferred to the 2nd U-boat Flotilla and left Kiel for the West Atlantic on 28 August 1944 for her first and only patrol. Stopping briefly in Bergen, Norway, for replenishment, she operated off the Canadian coast, damaging HMCS Magog on 14 October 1944 and the British steamer Fort Thompson on 2 November. Magog was towed back to port, but declared a constructive loss and decommissioned.

U-1223 arrived back in Kristiansand on Christmas Eve, 24 December 1944, and continued her journey to Flensburg, where she arrived three days later. Having been transferred to the 33rd U-boat Flotilla, she left Flensburg again for Königsberg on 5 January 1945, arriving there on the tenth. The U-boat experienced technical problems in the end of January 1945 and had to be towed into Stettin. From there she travelled under tow of U-1108 to Wesermünde, where she was decommissioned on 15 April 1945. Most of her crew was ordered to form a tank destroyer unit in Neustadt in Holstein under the command of the first watch officer.

When British forces closed in on the port, the U-boat was scuttled in position 53°32′N 8°35′E / 53.533°N 8.583°E / 53.533; 8.583 on 5 May 1945. Her wreck was later broken up for scrap.

Date	Ship Name	Nationality	Tonnage	Fate
14 October 1944	HMCS Magog	Royal Canadian Navy	1,370	Total loss
2 November 1944	Fort Thompson	United Kingdom	7,134	Damaged