USS Ruby (PY-21) was a converted yacht that patrolled with the United States Navy in World War II.

Placida—built as a diesel motor yacht in 1930 at Bath, Maine, by Bath Iron Works—was purchased by the Navy from H. G. Haskell on 19 June 1941; renamed Ruby and classified as a patrol vessel, PY-21, on 6 July 1941; converted at the Gibbs Gas Engine Company, Jacksonville, Florida; and commissioned on 23 September 1941.

[1] Initially assigned to the 6th Naval District at Charleston, South Carolina, for escort duty, Ruby was assigned to the Eastern Sea Frontier from 10 February 1942 to 18 January 1944, retaining her homeport of Charleston.

[1] Ruby decommissioned at Boston on 25 July 1945, was stricken from the List of Naval Vessels on 13 August 1945, and transferred to the Maritime Commission for disposal on 26 December, the day after Christmas.

[1] This article incorporates text from the public domain Dictionary of American Naval Fighting Ships.