The third USS Ellen (SP-1209) was a United States Navy patrol vessel in commission from 1917 to 1919.

A photograph of either Ellen or her sister ship Inca in 1917, prior to their United States Navy service; this photograph was used on data cards for both boats without distinguishing between them. Inca later served as patrol vessel USS Inca (SP-1212).
History
United States
Name	USS Ellen (1917–1918) USS SP-1209 (1918–1919)
Namesake	Ellen was her previous name retained; SP-1209 was her section patrol number
Builder	Herreshoff Manufacturing Company, Bristol, Rhode Island
Completed	1917
Acquired	21 July 1917
Commissioned	2 August 1917
Renamed	USS SP-1209 in 1918
Fate	Returned to owner April 1919 Sank en route to Puerto Rico in 1959 or 1960
Notes	Operated as civilian motorboat Ellen 1917 and from 1919
General characteristics
Type	Patrol vessel
Tonnage	23 tons
Length	62 ft 4 in (19.00 m)

Ellen was built as a civilian motorboat of the same name in 1917 by the Herreshoff Manufacturing Company at Bristol, Rhode Island, one of nine motorboats built to a common design for private owners in anticipation that their owners would make them available for naval service. The U.S. Navy acquired Ellen from her owner on 21 July 1917 for World War I service as a patrol vessel. She was commissioned on 2 August 1917 as USS Ellen (SP-1209).

Ellen operated on section patrol duty for the rest of World War I. She was returned to her owner in April 1919.

Ellen should not be confused with USS Ellen (SP-284), another patrol vessel in commission at the same time.