German submarine U-66 was a Type IXC U-boat of Nazi Germany's Kriegsmarine during World War II. The submarine was laid down on 20 March 1940 at the AG Weser yard at Bremen, launched on 10 October and commissioned on 2 January 1941 under the command of Kapitänleutnant Richard Zapp as part of the 2nd U-boat Flotilla.

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.