HMS Undaunted was an Arethusa class light cruiser, completed in August 1914. She spend almost the entire war with the Harwich Force, joining it as the leader of the 3rd Destroyer Flotilla at the end of August 1914.
On 17 October she led four destroyers of her flotilla against a force of four German destroyers off the Dutch coast. The four German destroyers (S 115, S 117, S 118 and S 119) were all sunk during the engagement.
In November 1914 she was part of the force mobilised in an attempt to catch the German ships that had raided Gorleston (part of Yarmouth). Unlike most of the British ships involved, she made contact with German cruisers, and attempted to draw then on to more powerful forces, but without success.
On 25 December 1914 she was part of a force of three cruisers, three seaplane carriers, nine seaplanes and associated destroyers and submarines that attacked the Cuxhaven Zeppelin base.
On 24 January 1915 she was present at the battle of Dogger Bank, but did not get involved in the main fighting. She then moved to the Irish Sea, with her destroyers, to undertake anti-submarine duties. On 9-12 February they escorted the Canadian division from Britain to France. On April 1915 she was badly damaged in a collision with the destroyer Landrail, the first of two collisions in her career.
In August 1915 she was transferred to the 9th Destroyer Flotilla. In the same month she took part in the hunt for the German minelayer Meteor, which ended with scuttling of the German ship.
The Undaunted was part of the force that covered the raid on Tondern (or Hoyen), on 24 March 1916. During the return trip the flagship, HMS Cleopatra, turned sharply to ram a German destroyer, and cut across the bows of the Undaunted. She was badly damaged in the collision, and could only make 6kts without damaging her bulkheads. For some time it looked as if the German High Seas Fleet would catch her, triggering a possible battle which was avoided when the Germans turned back. Jellicoe had to detach his Queen Elizabeth class battleships to protect the Undaunted as she made her way back to harbour. She reached safety at Seaham Harbour on 28 March, four days after the collision.
In April 1917 she was converted to carry 70 mines, but she didnít carry out any mine laying missions. On 5 June 1917 she was part of the British fleet that took part in the bombardment of the German base at Ostend. In October she was part of the force deployed in a attempt to find the German fleet that was actually attacking a Scandinavian convoy.
In November 1918 Undaunted left the Harwich Force, and joined the 7th Light Cruiser Squadron of the Grand Fleet, remaining there until March 1919. The next month she joined the Nore Reserve. At the start of 1921 she was reactivated to carry troops to the Mediterranean, before returning to the reserve.