SM UC-78 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) 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.