German submarine U-873 was a German long-range Type IXD2 U-boat of World War II. Following the surrender of Germany, the United States Navy studied U-873 to improve United States submarine designs. U-873 is remembered for the controversial treatment of its crew as prisoners of war and the death of commanding officer Kapitänleutnant Friedrich Steinhoff in a Boston jail cell. Six months after Steinhoff's death, his brother was one of the Operation Paperclip rocket scientists from Peenemünde arriving in the United States to work at White Sands Missile Range.

The Type IX U-boat was designed by Nazi Germany's Kriegsmarine in 1935 and 1936 as a large ocean-going submarine for sustained operations far from the home support facilities. Type IX boats were briefly used for patrols off the eastern United States in an attempt to disrupt the stream of troops and supplies bound for Europe. It was derived from the Type IA, and appeared in various sub-types.