First Lord of the Admiralty, or formally the Office of the First Lord of the Admiralty, was the title of the political head of the English and later British Royal Navy. He was the government's senior adviser on all naval affairs, responsible for the direction and control of the Admiralty, and also of general administration of the Naval Service of the Kingdom of England, Great Britain in the 18th century, and then the United Kingdom, including the Royal Navy, the Royal Marines, and other services. It was one of the earliest known permanent government posts. Apart from being the political head of the Naval Service the post holder was simultaneously the pre-eminent member of the Board of Admiralty. The office of First Lord of the Admiralty existed from 1628 until it was abolished when the Admiralty, Air Ministry, Ministry of Defence and War Office were all merged to form the new Ministry of Defence in 1964. Its modern-day equivalent is the Secretary of State for Defence.

In 1628, during the reign of Charles I, George Villiers, 1st Duke of Buckingham, Lord High Admiral of England, was assassinated and the office was placed in commission, under the control of a Board of Commissioners.

The first such First Lord of the Admiralty was Richard Weston, 1st Earl of Portland, who was appointed in 1628. The First Lord was not always a permanent member of the board until the Admiralty Department was established as an official government department in 1709 with the First Lord as its head; it replaced the earlier Office of the Admiralty and Marine Affairs. During most of the 17th century and the early 18th century, it was not invariable for the Admiralty to be in commission, so there are gaps in the list of First Lords, and a small number of First Lords were for a time Lord High Admiral.

After the Glorious Revolution, during the reign of William and Mary, Parliament passed the Admiralty Act 1690 (2 Will. & Mar. Sess. 2. c. 2), vesting in the commissioners the powers formerly held by the Lord High Admiral of England. and at this point became a permanent Cabinet position.

The Admiralty Commission was dissolved in 1701, but was reconstituted in 1709 on the death of Prince George of Denmark, who had been appointed Lord High Admiral. The office has been held in commission from that time onwards, however, except for a short period (1827–28) when the Duke of Clarence was Lord High Admiral. The Board of the Admiralty comprised a number of "Lords Commissioners" headed by a First Lord.

From the early 1800s the post was always held by a civilian (previously flag officers of the Royal Navy also held the post). In 1832 First Lord Sir James Graham instituted reforms and amalgamated the Board of Admiralty and the Navy Board. By the provisions of the Admiralty Act 1832 (2 & 3 Will. 4. c. 40), two lords in committee could legalise any action of the board.

In 1868 Prime Minister, William Gladstone appointed Hugh Childers First Lord, who would introduce a new system at the Admiralty. However these changes restricted communication between the board members who were affected by these new regulations, and the sittings of the Board were discontinued altogether. This situation described was further exacerbated by the disaster of HMS Captain in 1870, a poorly-designed new vessel for the navy.

The responsibility and powers of the First Lord of the Admiralty were laid down by an Order in Council dated 14 January 1869, and a later Order (19 March 1872) made the First Lord responsible to the Sovereign and to Parliament for all the business of the Admiralty. However, by describing the Lords of the Admiralty as the "assistants" of the First Lord, and by specifically defining their duties, this had, in fact, partially disabled the collective power of the Board.