Admiral Sir Sydney Colpoys Dacres GCB (9 January 1804 – 8 March 1884) was an officer of the Royal Navy who saw service during the Greek War of Independence, when he was involved in an attack on the Turkish forces at Morea, and later during the Crimean War. Born into a substantial naval dynasty during the Napoleonic Wars, he eventually rose to the rank of Admiral and became First Naval Lord. His only significant action as First Naval Lord was to press for the abolition of masts. He went on to be Visitor and Governor of Greenwich Hospital.
Dacres then moved ashore, becoming Captain-Superintendent of the Royal Hospital Haslar and the Royal Clarence (Gosport) Victualling Yard in July 1855, a post he held until 25 June 1858, the date he was promoted to rear-admiral. He became Captain of the fleet aboard HMS Marlborough to the commander-in-chief of the Mediterranean Fleet on 12 September 1859, serving under Vice-Admiral Arthur Fanshawe, and then Vice-Admiral William Fanshawe Martin. In April 1861 he flew his flag on HMS Caesar then from 16 December 1861 Dacres became second in command in the Mediterranean, flying his flag aboard HMS Edgar. He then became commander in chief of the Channel Squadron on 24 April 1863, a post he held until June 1866 and during which he oversaw the integration of the new ironclads into the fleet. He was appointed a Knight Commander of the Bath on 28 March 1865, and promoted to vice-admiral on 17 November 1865, while in command of the Channel Squadron.
Dacres became a Commissioner of the Admiralty and Second Naval Lord on 12 July 1866, rising to be the First Naval Lord on 18 December 1868. He was promoted to admiral on 1 April 1870, and was appointed a Knight Grand Cross of the Bath on 20 May 1871. His only significant action as First Naval Lord was to press for the abolition of masts. He stepped down as First Naval Lord on 27 November 1872, becoming Visitor and Governor of Greenwich Hospital on 2 December that year. He was placed on the retired list on 10 January 1874.