"S18" remained a member of the 13th half-flotilla of the 7th Flotilla, part of the German High Seas Fleet, at the outbreak of the First World War in August 1914. The 7th Torpedo Boat Flotilla supported the Raid on Yarmouth on 3 November 1914 and the Raid on Scarborough, Hartlepool and Whitby on 16 December 1914. The flotilla continued to support the High Seas Fleet in 1915, sortieing into the German Bight with the 5th and 9th Flotilla on 19/20 January and escorting a sortie of the fleet's battleships to off Terschelling on 29/30 March. "S18", as part of the 7th flotilla, was part of the High Seas Fleet when it sailed to cover the Lowestoft Raid on 24–25 April 1916.