The Loch class was a class of anti-submarine (A/S) frigate built for the Royal Navy and her Allies during World War II. They were an innovative design based on the experience of three years of fighting in the Battle of the Atlantic and attendant technological advances. Some shipyards had trouble building these larger ships, which led to widespread use of the Castle-class corvette, introduced around the same time.

The Lochs were based upon the hull of the preceding River class with increased sheer and flare to improve seakeeping and modified to suit it to mass pre-fabrication, with sections riveted or welded together at the shipyard. Accordingly, as many curves as possible were eliminated, producing a noticeable kink in the main deck where the increased sheer forwards met the level abreast the bridge.

The fighting capability of the Loch class was a great jump forward, being based around the new Squid ahead-throwing A/S mortar. Previously, escorts had attacked with depth charges, requiring the attacking vessel to pass over the submerged submarine and drop warheads over the stern. The ahead-looking ASDIC (sonar) set lost contact at the vital moment before the weapons were launched, allowing a skilled submarine commander to take evasive action. Squid lobbed three 12 in (300 mm), 390 lb (180 kg) warheads up to 600 yd (550 m) ahead of the launching vessel falling in a triangular pattern with sides of 40 yd (37 m). As the clockwork fuses detonated the warheads at a depth determined by the ships' ASDIC set, the Squid attacked in full ASDIC contact giving greater accuracy and lethality. Two mortars were carried, with the bombs set to explode at different depths to create a convergent pressure wave to crush the hull of a submarine. The first successful "kill" was by Loch Killin on 31 July 1944, sinking the U-333. Such was the utility of Squid that depth charge carriage was reduced to only 15, with one rack and a pair of throwers being carried at the stern, resulting in a clear quarterdeck compared to the eight throwers, two racks and stowage for 100 plus charges in the Rivers.

The weapons outfit was completed with a QF 4 in (100 mm) Mark V gun forwards. An oversight in the River design had been to place the gun in the favourable position forward on the shelter deck, with the ahead throwing Hedgehog on the main deck, exposed to the worst of the seas breaking over the fo'c'sle, consequently this was reversed in the Lochs; the Squids being on the shelter deck behind protective bulwarks. For anti-aircraft (A/A) defence, a quadruple mounting Mark VII QF 2-pounder was shipped aft along with up to twelve 20 mm Oerlikon guns, in two twin powered mounts Mark V in the bridge wings and eight single pedestal mounts Mark III. Loch Arkaig, Loch Craggie, Loch Eck, Loch Glendhu, Loch Tralaig and the South African Navy ships HMSAS Good Hope, Natal and Transvaal carried single Bofors 40 mm L/60 gun mounts in lieu of the twin Oerlikons.

The Lochs carried Radar Type 277. This set used the cavity magnetron to transmit on centimetric wavelengths for target indication, excelling at picking out small targets such as a submarine periscope or snorkel from the surface clutter. The increased weight of the stabilised antenna array and the carriage of HF/DF at the masthead meant that a lattice mast was stepped in lieu of the traditional tripod or pole. Some vessels completed with Radar Type 271 or 272 and the associated "lantern" radome until Type 277 became available. ASDIC Type 144 was carried for search and attack functions with Type 147B used for depth finding.

As with the previous wartime escort designs, mercantile machinery was adopted to speed construction, with a pair of 4-cylinder vertical triple-expansion reciprocating engines. Loch Arkaig and Loch Tralaig were fitted with Parsons single reduction steam turbines to establish the feasibility of such an installation, but it was not possible to provide enough sets of turbines for all vessels. One advantage of the use of mercantile machinery was that it was instantly familiar to the mainly reservist and volunteer (RNR and RNVR) crews who manned the ships.

Of the one hundred and ten vessels ordered, twenty-eight were built as frigates, entering service from 1944. Another two – Loch Assynt and Loch Torridon – were converted while building and completed as Coastal Forces Depot Ships, being renamed HMS Derby Haven and HMS Woodbridge Haven. Due to a need in 1944 for a version fitted as anti-aircraft vessels with the British Pacific Fleet, twenty-six units were authorised for completion to a modified design labelled the Bay-class frigate and were renamed. A further fifty-four Loch-class vessels were cancelled in 1945.

Of the twenty-eight Loch-class frigates completed as such, Loch Achanalt, Loch Alvie and Loch Morlich were transferred to Canada in 1943 but retained their Royal Navy names and were returned after the war; a fourth ship – Loch Fionn – was also earmarked for transfer but was retained by the Royal Navy as a Bay-class frigate. Loch Ard, Loch Boisdale and Loch Cree were transferred to South Africa as Transvaal, Good Hope and Natal respectively on completion. In 1948, six vessels, including two of the ex-Royal Canadian Navy trio, were refitted from reserve and transferred to New Zealand. During the Korean War, the Royal Navy reactivated several vessels and transferred them to the Mediterranean where they released Ch-class destroyers for war duties. In 1964 Loch Insh was transferred to Malaysia.