Admiral of the Fleet Rosslyn Erskine Wemyss, 1st Baron Wester Wemyss,, known as Sir Rosslyn Wemyss between 1916 and 1919, was a Royal Navy officer. During the First World War he served as commander of the 12th Cruiser Squadron and then as Governor of Moudros before leading the British landings at Cape Helles and at Suvla Bay during the Gallipoli campaign. He went on to be Commander of the East Indies & Egyptian Squadron in January 1916 and then First Sea Lord in December 1917, in which role he encouraged Admiral Roger Keyes, Commander of the Dover Patrol, to undertake more vigorous operations in the Channel, ultimately leading to the launch of the Zeebrugge Raid in April 1918.

Admiral of the Fleet is a five-star naval officer rank and the highest rank of the Royal Navy, formally established in 1688. The five-star NATO rank code is OF-10, equivalent to a field marshal in the British Army or a Marshal of the Royal Air Force. Apart from honorary appointments, no new admirals of the fleet have been named since 1995, and no honorary appointments have been made since 2014.