An unprotected cruiser was a type of naval warship that was in use during the early 1870s Victorian or pre-dreadnought era. The name was meant to distinguish these ships from “protected cruisers”, which had become accepted in the 1880s. A protected cruiser did not have side armor on its hull like a battleship or “armored cruiser” but had only a curved armored deck built inside the ship — like an internal turtle shell — which prevented enemy fire penetrating through the ship down into the most critical areas such as machinery, boilers, and ammunition storage. An unprotected cruiser lacked even this level of internal protection. The definitions had some gray areas, because individual ships could be built with a protective deck that did not cover more than a small area of the ship, or was so thin as to be of little value. The same was true of the side armor on some armored cruisers. An unprotected cruiser was generally cheaper and less effective than a protected cruiser, while a protected cruiser was generally cheaper and less effective than an armored cruiser, with some exceptions in each case.

Protected cruisers, a type of cruising warship of the late-19th century, gained their description because an armoured deck offered protection for vital machine-spaces from fragments caused by shells exploding above them. Protected cruisers resembled armored cruisers, which had in addition a belt of armour along the sides.