The sixth USS Niagara (SP-136), later PY-9, was a United States Navy patrol vessel in commission from 1918 to 1931 and which served during World War I. Niagara was a steam yacht built in 1898 by Harlan and Hollingsworth, Wilmington, Delaware.

[2] Niagara departed New York on 21 May 1918 as escort for a merchant convoy bound for Bermuda and the Azores.

On 5 September 1918 she stood out of Grassy Bay to rescue and tow in the merchant sloop Gauntlet, which was adrift after her sails had been carried away in a storm.

[2] Reclassified PY-9 on 17 July 1920, she continued patrols in the Caribbean Sea as a unit of the special service squadron until decommissioning at Philadelphia, Pennsylvania, on 21 April 1922.

[4] Originally, Niagara had a Welte Style 6 Concert Orchestrion which went through two decks (as pictured below - only the lower portion of the instrument is visible in the second row center photograph); Gould replaced this in 1912 with a Welte Philharmonic Organ.