The River class was a class of 151 frigates launched between 1941 and 1944 for use as anti-submarine convoy escorts in the North Atlantic. The majority served with the Royal Navy and Royal Canadian Navy (RCN), with some serving in the other Allied navies: the Royal Australian Navy (RAN), the Free French Naval Forces, the Royal Netherlands Navy and, post-war, the South African Navy.

The Royal Navy placed the first orders in 1940, and the vessels were named after rivers in the United Kingdom, giving the name to the class. In Canada, they were called after towns and cities, although they retained the same designation. Originally called a "twin-screw corvette", the name "frigate" was suggested by Vice-Admiral Percy W. Nelles of the Royal Canadian Navy. Canada originally ordered the construction of 33 frigates in October 1941. The design was too big for the locks on the Lachine Canal so it was not built by the shipyards on the Great Lakes and therefore all the frigates built in Canada were built in dockyards along the West Coast or along the St. Lawrence River below Montreal. In all, Canada ordered the construction of 70 frigates, including ten for the Royal Navy, which transferred two (USS Asheville and USS Natchez) to the United States Navy. These served as the basis of the US Navy Tacoma class frigate series. Twelve were built in Australia for the RAN (four to a modified design).

After World War II, they found employment in many other navies the world over; several RCN ships were sunk as breakwaters. One, HMCS Stormont, was purchased by Aristotle Onassis and converted into the luxury yacht Christina O.

The River-class ships were designed by naval engineer William Reed, of Smith's Dock Company of South Bank-on-Tees, to have the endurance and anti-submarine capabilities of the Black Swan-class sloops, while being quick and cheap to build in civil dockyards using the machinery (e.g. reciprocating steam engines instead of turbines) and construction techniques pioneered in the building of the Flower-class corvettes. Its purpose was to improve on the convoy escort classes in service with the Royal Navy, including the Flower class.

Improvements over the corvette design included markedly better accommodation. The twin engines gave only 3 knots (3 mph; 6 km/h) more speed but extended the range of the ship to nearly double that of a corvette to 7,200 nmi (13,300 km; 8,300 mi) at 12 kn (22 km/h; 14 mph). Among other lessons applied to the design was armament better designed to combat U-boats, including a twin 4 in (100 mm) mount forward and 12-pounder [76 millimetres (3.0 in)] aft. Fifteen Canadian frigates were initially fitted with one 4 in (100 mm) gun forward but with the exception of HMCS Valleyfield, they were all eventually upgraded to the twin mount. For underwater targets, it was equipped with a Hedgehog anti-submarine mortar, depth charge rails and four side-mounted throwers aft for a 10-charge pattern (some had 8 throwers for a 14-charge pattern for a brief period until this was abandoned).

River-class frigates were the first Royal Canadian Navy warships to carry the 147B Sword horizontal fan-beam active sonar transmitter, in addition to the regular ASDIC. This allowed the ship to maintain contact with targets even while firing, unless a target was struck. Better radar and radio direction-finding equipment enhanced the RCN's ability to locate and track enemy submarines over previous classes. The River-class design was used as the basis for the United States Navy Tacoma class (which served in the Royal Navy as the Colony class); the hull design was later elaborated into the Loch class and subsequently the Bay class.

Two hundred and forty-three frigates were built in Britain, Canada and Australia for seven navies during World War II.

On display in Brisbane, Australia is HMAS Diamantina, the last complete River-class frigate, preserved at the Queensland Maritime Museum.