HMS Princess Margaret was a minelayer operated by the British Royal Navy during and after the First World War. She was built by the Scottish shipbuilder William Denny for the Canadian Pacific Railway as a liner to serve on the Pacific West Coast, and as such was powered by geared steam turbines, giving a speed of 23 knots (43 km/h; 26 mph).

The outbreak of war caused her to be taken over by the Royal Navy and to be converted to a minelayer, carrying as many as 500 mines. She was widely used for minelaying in the North Sea and English Channel during the war, which she survived, having laid the most mines of any Royal Navy ship during the war. She remained in Royal Navy service following the end of the war, taking part in the British intervention in the Russian Civil War. She was sold for scrap in 1929.

In 1914, two fast passenger ships, Princess Irene and Princess Margaret, were being built by the Scottish shipbuilder William Denny for the Princess fleet of the Canadian Pacific Railway Coast Service to use in its service between Vancouver and Seattle. Princess Margaret was launched at Denny's Dunbarton shipyard on 24 June 1914.

At the outbreak of the First World War, the Royal Navy's minelaying capability was limited to seven old Apollo class cruisers. These ships could carry 100–140 mines and although originally designed for a speed of 20 knots (23 mph; 37 km/h), by 1914 they were only capable of 15 knots (17 mph; 28 km/h).

To improve the minelaying capability of the Royal Navy, Princess Irene and Princess Margaret were requisitioned and converted to minelayers. They were 395 feet 6 inches (120.55 m) long overall with a beam of 54 feet (16.46 m) and a draught of 16 feet 9 inches (5.11 m). The ships had a gross register tonnage of 5934 t. Ten Babcock & Wilcox water-tube boilers fed steam at 202 pounds per square inch (1,390 kPa) to geared steam turbines driving two shafts. The machinery was rated at 15,000 shaft horsepower (11,000 kW) giving a speed of 22.5 knots (25.9 mph; 41.7 km/h). They had a crew of 225.

Armament consisted of two 4.7 in (120 mm) guns, two 12-pounder (76 mm) guns, two 6-pounder (57 mm) anti-aircraft guns and one 2-pounder pom-pom anti-aircraft autocannon. They could carry up to 500 mines.

Princess Margaret was commissioned on 26 December 1914. By March 1915, she was listed as being a member of the Minelaying Squadron. On 8 May 1915 Princess Margaret and Princess Irene laid a minefield northwest of Heligoland, with Princess Margaret laying 490 mines. On 16 August 1915, Princess Margaret, escorted by two divisions of the 10th Destroyer Flotilla, eight destroyers in total, was tasked with laying a minefield on the Amrum Bank. At about 8:45 pm five patrolling German destroyers encountered the British force near the Horns Reefs light vessel. The German destroyer B98 fired two torpedoes, one of which stuck the British destroyer Mentor. Princess Margaret turned away to avoid the attack, with the rest of the British destroyers (most of which had not spotted the German ships and thought that Mentor had struck a mine) following. The German force also turned away, and Mentor, which had her bow blown off, was left by herself to make her way back to base. Despite the damage, Mentor made it safely back to Harwich. On 10 September 1915 Princess Margaret and the minelayers Angora and Orvieto set out from the Humber with a close escort of six destroyers and with heavy distant covering forces (including most of the rest of the Harwich Force and the Battle Cruiser Force out of Rosyth) on Operation CY, another attempt to lay a minefield off the Amrum Bank. This time the Germans did not interfere, and the three minelayers laid a total of 1,450 mines on the night of 10/11 September. The German light cruiser Graudenz struck a mine in this minefield on the night of 21/22 April 1916. On the night of 8/9 November 1915 Princess Margaret and Angora laid another field of 850 mines to replace the field laid in September, which had been discovered by the Germans. While the minefield was laid successfully, the destroyer Matchless, part of the covering force, struck a German mine on the return journey to Harwich.

By March 1916, Princess Margaret was listed as flagship of the Minesweeping Squadron. On 20 March 1916, Princess Margaret, Orviedo, Paris and Biarritz laid mines in the North Sea approaches to the English Channel and Thames, between the North Hinder and Galloper light ships. On 24 April 1916, Princess Margaret, together with Biarritz, Orvieto and Paris, laid a large minefield off the Belgian coast, which was supplemented by mine-nets laid by trawlers, in order to constrain the activities of German submarines based in Flanders. On 3–4 May 1916, the British launched Operation XX, an attempt to lure the German High Seas Fleet out to sea where it could be attacked, and to draw German naval forces away from the Baltic. The seaplane carriers Vindex and Engadine would launch an air attack on the German Airship base at Tondern. Minefields would be laid at the exits of the swept channels through the Heligoland Bight, which any German forces sortieing in response would have to pass through, with 10 submarines waiting off the Horns Reef and off Terschelling. The Battle Cruiser Force would be waiting 35 nautical miles (65 km; 40 mi) off Terschelling, while the battleships of the Grand Fleet would provide distant cover. Princess Margaret set off from the Humber on 3 May 1916, escorted by the destroyers Lark and Lucifer, bound for the western end of the Heligoland Bight. Princess Margaret left her escorts 60 nautical miles (110 km; 69 mi) west of the proposed minefield, continuing alone, and successfully laid 530 mines. The air raid itself was a failure, with only one aircraft managing to attack its target, and the hoped for confrontation between the British and German fleets did not occur, although one German airship, L 7, was shot down by the British light cruisers Galatea and Phaeton.