SM UC-78 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 8 December 1916. She was commissioned into the German Imperial Navy on 10 January 1917 as SM UC-78. In twelve patrols UC-78 was credited with sinking 1 warship, either by torpedo or by mines laid. UC-78 was rammed and sunk by the British steamer Queen Alexandra west of Cherbourg on 9 May 1918.
A Type UC II submarine, UC-78 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 30 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-78 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.