"Roon" participated in the Battle of the Gulf of Riga in August 1915 and bombarded Russian positions at Zerel on the Sworbe Peninsula in company with the armored cruiser . The two cruisers surprised a group of Russian destroyers and damaged one of them before the Russians fled. By late 1915, the increased threat of British submarines, demonstrated by the sinking of in October, convinced the German naval command to withdraw the remaining armored cruisers still in service (by this point, just "Roon" and "Prinz Heinrich"). "Roon" was disarmed in 1916 and used as a training ship and a floating barracks. Design work commenced in 1916 to convert the ship into a seaplane tender; work was planned to last from 1917 to 1918 during a period of 20 months. The ship was stricken from the naval register on 25 November 1920 and scrapped the following year at Kiel-Nordmole.