It is believed that the Spanish Armada fleet (the 130 ships sent by Philip II of Spain to attack England) of 1587 and 1588 included hospital ships. England's first recorded hospital ship was the Goodwill in 1608; however, it was not until after 1660 that the Royal Navy made it a regular practice to set aside ships for hospital use.
The U.S. government outfitted six hospital ships, some of which were permanent members of its fleet during the Spanish-American War of 1898. In August 1916, Congress authorized the construction of the hospital ship the U.S.S. Relie/. (U.S.S. stands for "United States Ship" or "United States Steamer.") It was launched in 1919 and delivered to the navy in December 1920.
Sources: Kane, Joseph N. Famous First Facts, 4th ed., p. 575; Plumridge, John H. Hospital Ships and Ambulance Trains, pp. 13, 31.