USS Elf (SP-81) was an armed motorboat that served in the United States Navy as a patrol vessel from 1917 to 1919. Elf was a civilian motorboat completed in early 1917 by . In early May 1917, the U.S. Navy acquired her under a free lease from her owner, Louis H. Eisenlohr of Philadelphia, Pennsylvania, for World War I service as a patrol boat. She was commissioned as USS Elf (SP-81) on 3 May 1917. Elf performed patrol duty in the 4th Naval District until July 1918, when she was transferred to the 7th Naval District.