The S class (initially known as the Modified Trenchant class) was a class of 67 destroyers ordered for the Royal Navy in 1917 under the 11th and 12th Emergency War Programmes. They saw active service in the last months of the First World War and in the Russian and Irish Civil Wars during the early 1920s. Most were relegated to the reserve by the mid-1920s and subsequently scrapped under the terms of the London Naval Treaty. Eleven survivors saw much action during the Second World War.

In early 1917, the First World War had been going on for two and a half years. Despite the disappointing outcome of the Battle of Jutland the previous year, the British Grand Fleet, consisting of battleships, cruisers, and destroyers and based in northern Scotland, was successfully confining the German surface navy to the German Bight, while enforcing a blockade of German maritime trade with the wider world. In southern North Sea, the Harwich Force and the Dover Patrol, both consisting of cruisers and destroyers, maintained control of the eastern approaches to the English Channel and the Thames Estuary and safeguarded British communications with France. German submarine attacks on British trade became increasingly effective during the autumn of 1916, and unrestricted submarine warfare was soon to begin; finding effective countermeasures was increasingly taxing for the Admiralty. In the Mediterranean, the Royal Navy provided support to the French and Italian Fleets countering the threat posed by the Austro-Hungarian and Turkish fleets. All of these diverse naval commitments placed huge demands on the Royal Navy's resources; in particular, the need for large numbers of destroyers was pressing.

Since mid-1916, destroyer production had concentrated on the large and powerful V and W classes, intended to match large German destroyers reported to be under construction. However, the Admiralty had come to appreciate that these intelligence reports had been overstated, thus the next orders could revert to the smaller destroyers of the Modified R class of March 1916, which could be built in large numbers quickly and cheaply. The resultant design, formulated in February 1917, incorporated some wartime lessons and suggestions from destroyer officers serving with the fleet, thus was identified as a new class.

The new class had two funnels, a long forecastle and a tall bridge, which unusually, was located behind the break in the main deck. Following consultations with sea-going officers, several novel features were included, principally to enhance sea-keeping capabilities in rough weather and battle-worthiness in night action. Consideration was given to having a well deck forward of the bridge with an additional pair of torpedo tubes, in the fashion of contemporary German ocean-going torpedo boats, but this arrangement was rejected because it would be detrimental to ship handling in rough weather. The design had a heavily raked stem and sheer forward, a slight turtleback on the fo'c's'le deck, and a rounded bridge front, intended to deflect waves. Two single 18-inch (450 mm) torpedo tubes were fitted under the bridge on single rotating mounts port and starboard, intended to be fired directly by the commanding officer with toggle ropes during night actions, and the searchlight was relocated aft, mounted on top of the aft torpedo tubes (this to avoid the bridge officers being blinded by its glare during night actions). The single 18-inch torpedo tubes proved to be of little use, and were soon removed in the majority of vessels as a weight-saving measure (only Tara, Tintagel, and Trojan retained theirs).

The S class was built in two batches, the first 33 ordered on 9 April 1917 and the second batch of 36 in June 1917, respectively. Most were built to the design prepared by the Admiralty ("Admiralty S class"), finalized on 3 July 1917. As was common practice during the First World War, the Admiralty allowed two specialist destroyer builders, Thornycrofts and Yarrows a free hand to develop their own designs based on the current Admiralty designs, which other yards were contracted to build. Generally, these "specials" had considerably higher speeds; this performance enhancement stemmed largely from the higher quality of workmanship practiced by these specialist builders, rather than due to deficiencies of the Admiralty designs. Thus the seven vessels ordered from Yarrow were built to a distinct design ("Yarrow S class"); similarly the five ordered from Thornycroft were the "Thornycroft S class".

Most of the Admiralty S class had Brown-Curtis single-reduction geared turbines; seven vessels (Steadfast, Sterling, Stonehenge, Stormcloud, Tilbury, Tintagel, and Strenuous) had Parsons Impulse-Reaction turbines, instead. Three Yarrow boilers (maximum pressure 250 psi (1,700 kPa)) were fitted in all except the vessels built by Whites (which had White-Forster boilers). The highest trial speed of the Admiralty S class was 35.837 knots (66.370 km/h; 41.240 mph) recorded by Senator (with 28,076 shaft horsepower (20,936.3 kW), 360.1 rpm) at a displacement of 1,019 tons.

Senator conducted trials in June 1918 to test the feasibility of a flying off platform aft, with the intent of launching an aircraft with the ship proceeding full speed astern; however, the amount of spray rendered this proposal unworkable. Later (c. 1927–28) both Stronghold and Thanet were fitted with a cordite catapult for launching aircraft from their fo'c's'le (the forward gun was removed); they were used to test the Larynx, an early experimental type of cruise missile.

Various S-class destroyers were used at different times during the 1920s and 1930s as attendant vessels for aircraft carriers: Searcher in the Mediterranean Fleet and Tyrian, Sesame, Sturdy, and Stronghold in the Home Fleet. Their armament was removed (in order to lighten the ship for maximum speed), and a davit installed on the fo'c's'le for recovery of ditched aircraft.