USS Portsmouth was a Cleveland-class light cruiser of the United States Navy, which were built during World War II.

The class was designed as a development of the earlier Brooklyn-class cruisers, the size of which had been limited by the First London Naval Treaty.

The start of the war led to the dissolution of the treaty system, but the dramatic need for new vessels precluded a new design, so the Clevelands used the same hull as their predecessors, but were significantly heavier.

Following the start of World War II in September 1939, Britain announced it would suspend the treaty for the duration of the conflict, a decision the US Navy quickly followed.

Though still neutral, the United States recognized that war was likely and the urgent need for additional ships ruled out an entirely new design, so the Clevelands were a close development of the earlier Brooklyn-class cruisers, the chief difference being the substitution of a two-gun 5 in (127 mm) dual-purpose gun mount for one of the main battery 6 in (152 mm) gun turrets.

Rated at 100,000 shaft horsepower (75,000 kW), the turbines were intended to give a top speed of 32.5 knots (60.2 km/h; 37.4 mph).

[2] The ship's belt armor ranged in thickness from 3.5 to 5 in (89 to 127 mm), with the thicker section amidships where it protected the ammunition magazines and propulsion machinery spaces.

She was thereafter based in Norfolk, Virginia; by that time, World War II had ended, precluding any significant wartime service.

Portsmouth embarked on a goodwill cruise to visit various ports in Africa, including Cape Town, South Africa; Lagos, Nigeria; Freetown, Sierra Leone; Monrovia, Liberia; Dakar, French Senegal; and Casablanca, French Morocco.