SM UC-66 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy () during World War I. The U-boat was ordered on 12 January 1916 and was launched on 15 July 1916. She was commissioned into the German Imperial Navy on 14 November 1916 as SM UC-66. In five patrols UC-66 was credited with sinking 32 ships, either by torpedo or by mines laid. UC-66 was sunk by HM seaplane No. 8656, a Curtiss Model H-12, off the Isles of Scilly on 27 May 1917. The wreck was found by divers in 2009. This is a notable early aircraft success against a U-boat.
A Type UC II submarine, UC-66 had a displacement of when at the surface and while submerged. She had a length overall of , a beam of , and a draught of . The submarine was powered by two six-cylinder four-stroke diesel engines each producing (a total of ), two electric motors producing , and two propeller shafts. She had a dive time of 48 seconds and was capable of operating at a depth of .
The submarine had a maximum surface speed of and a submerged speed of . When submerged, she could operate for at ; when surfaced, she could travel at . UC-66 was fitted with six mine tubes, eighteen UC 200 mines, three torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one Uk L/30 deck gun. Her complement was twenty-six crew members.