The Implacable-class aircraft carrier consisted of two aircraft carriers built for the Royal Navy during World War II. Derived from the design of the Illustrious class, they were faster and carried more aircraft than the older ships. They were initially assigned to the Home Fleet when completed in 1944 and attacked targets in Norway as well as the German battleship Tirpitz. Subsequently, they were assigned to the British Pacific Fleet (BPF).

Indefatigable was the first ship to go to the Pacific and attacked Japanese-controlled oil refineries in Sumatra en route. She participated in Operation Iceberg, the invasion of Okinawa in March–April 1945. Implacable's arrival in the Pacific was delayed by a refit and she did not begin operations against the Japanese until June. The sister ships participated in the attacks on the Japanese Home Islands in July and August. Indefatigable was the only carrier chosen to continue operations after most of the BPF withdrew to prepare for further operations in early August. After the Japanese formal surrender in September, Implacable ferried Allied troops and prisoners of war back to Australia and Canada for the rest of the year.

The sisters returned home in 1946; Indefatigable was used for the rest of the year to transport troops before being placed in reserve in 1947 and Implacable became the training carrier for Home Fleet. Indefatigable was converted into a training ship and reactivated in 1950 for service with the Home Fleet. Implacable was relegated to the reserve that same year and modified into a training ship in 1952. The sisters were scheduled for modernisation during the mid-1950s, but it was cancelled as the modernisation of Victorious before them proved expensive and lengthy. The sisters were decommissioned in 1954 and sold for scrap in 1955–1956.

The Implacable class had its origin as an improved version of the Illustrious-class aircraft carriers for the 1938 Naval Programme while still remaining within the 23,000 long tons (23,000 t) available from the tonnage allowed by the Second London Naval Treaty. The initial change was to increase the carriers' speed to no less than 32 knots (59 km/h; 37 mph) which required the addition of a fourth steam turbine and associated propeller shaft. Offsetting the additional weight of the machinery meant reductions in armour thicknesses in the hangar deck and the bulkheads at the ends of the hangar. At the same time the Director of Naval Construction (DNC) was developing a different modified Illustrious design (Design D) to carry an additional dozen aircraft (a total of 48) in a lower hangar that also incorporated the additional machinery of the initial design with the sacrifice of even more armour. Hangar height was initially planned as 13 feet 6 inches (4.1 m) in the upper hangar to accommodate the new Fairey Albacore torpedo bomber and 16 feet (4.9 m) in the lower hangar to accommodate taller amphibious aircraft, but a later change in policy raised the upper hangar height to 14 feet (4.3 m). Design D was submitted to the Board of Admiralty on 2 August 1938 and approved on 17 November. In April 1939 the lower hangar's height was reduced to 14 feet to compensate for the thickening of the hangar side armour to 2 inches (51 mm) and the idea of carrying amphibians in the hangar was abandoned.

The Implacable-class ships were 766 feet 6 inches (233.6 m) long overall and 730 feet (222.5 m) at the waterline. Their beam was 95 feet 9 inches (29.2 m) at the waterline and they had a draught of 29 feet 4 inches (8.9 m) at deep load. The ships were significantly overweight and displaced 32,110 long tons (32,630 t) at deep load. Their complement was approximately 2,300 officers and enlisted men in 1945. They had metacentric heights of 4.06 feet (1.2 m) at light load and 6.91 feet (2.1 m) at deep load as completed.

The ships were fitted with four Parsons geared steam turbines, each driving one shaft using steam supplied by eight Admiralty 3-drum boilers. The turbines were designed to produce a total of 148,000 shaft horsepower (110,000 kW), enough to give them a maximum speed of 32.5 knots (60.2 km/h; 37.4 mph). On their sea trials, the ships reached speeds of 31.89–32.06 knots (59.06–59.38 km/h; 36.70–36.89 mph) with 150,935–151,200 shp (112,552–112,750 kW). The Implacable class carried a maximum of 4,690–4,810 long tons (4,770–4,890 t) of fuel oil which gave them a range of 6,720–6,900 nautical miles (12,450–12,780 km; 7,730–7,940 mi) at 20 knots (37 km/h; 23 mph).

The 760-foot (231.6 m) armoured flight deck had a maximum width of 102 feet (31.1 m). The arrestor cables, crash barricades, aircraft catapult and lifts were designed to handle aircraft up to 20,000 pounds (9,100 kg) in weight. The carriers were fitted with nine arrestor cables aft that were designed to stop landing, at speeds of up to 60 knots (110 km/h; 69 mph). They were backed up by three crash barricades to prevent landing aircraft from crashing into aircraft parked on the ship's bow. In case of damage to the rear flight deck, the Implacable-class ships also mounted three additional forward arrestor cables to permit aircraft to land over the bow. A single BH3 hydraulic catapult was fitted on the forward part of the flight deck to launch 20,000-pound aircraft at 56 knots (104 km/h; 64 mph); lighter aircraft could be launched at a maximum speed of 66 knots (122 km/h; 76 mph). The ships were equipped with two lifts on the centreline, the forward of which measured 45 by 33 feet (13.7 by 10.1 m) and served only the upper hangar, and the aft lift (45 by 22 feet (13.7 by 6.7 m)) which served both hangars. The upper hangar was 458 feet (139.6 m) long and the lower hangar was 208 feet (63.4 m) long; both had a uniform width of 62 feet (18.9 m). Both hangars had a height of only 14 feet which precluded storage of Lend-Lease Vought F4U Corsair fighters as well as many post-war aircraft and helicopters. In case of fire the upper hangar could be divided by two fire curtains and the lower hangar had one fire curtain. Designed to stow 48 aircraft in their hangars, the use of a permanent deck park allowed the Implacable class to accommodate up to 81 aircraft. The crewmen, maintenance personnel and facilities needed to support these additional aircraft were housed in the lower hangar. The ships were provided with 94,650 imperial gallons (430,300 L; 113,670 US gal) of petrol, only enough for approximately five sorties per aircraft.

The ships' main armament consisted of sixteen QF 4.5-inch (110 mm) Mark II dual-purpose guns in eight powered RP 10 Mk II** twin-gun turrets, four in sponsons on each side of the hull. Unlike the Illustrious-class ships, the roofs of the gun turrets were flat and flush with the flight deck. The gun had a maximum range of 20,760 yards (18,980 m) at an elevation of +45° and a ceiling of 41,000 feet (12,000 m). Their light anti-aircraft defences included five octuple mounts for QF 2-pounder ("pom-pom") anti-aircraft guns, two on the flight deck forward of the island, one on the aft part of the island and two in sponsons on the port side of the hull. A single quadruple 2-pounder mount was also fitted on the port side of the hull. The 2-pounder gun had a maximum range of 6,800 yd (6,200 m). The two ships were also fitted with approximately sixty Oerlikon 20 mm autocannon in varying numbers of single and twin-gun mounts. These guns had a maximum range of 4,800 yd (4,400 m), but many were replaced by 40 mm Bofors AA guns when the ships were transferred to the Pacific War as the 20 mm shell was unlikely to destroy a kamikaze before it hit the ship. The Bofors gun had a maximum range of 10,750 yd (9,830 m). Two additional quadruple "pom-pom" mounts were added to Implacable before she joined the British Pacific Fleet in 1945. After the war, more Oerlikons were exchanged for Bofors guns. By April 1946, the sisters had 11–12 Bofors guns and 19–30 Oerlikons each.