The Royal Navy (RN) is the naval warfare force of the United Kingdom. It is a component of His Majesty's Naval Service, and its officers hold their commissions from the King. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against France. The modern Royal Navy traces its origins to the English Navy of the early 16th century; the oldest of the UK's armed services, it is consequently known as the Senior Service.

From the early 18th century until the Second World War, it was the world's most powerful navy. The Royal Navy played a key part in establishing and defending the British Empire, and four Imperial fortress colonies and a string of imperial bases and coaling stations secured the Royal Navy's ability to assert naval superiority. Following World War I, it was significantly reduced in size. During the Cold War, the Royal Navy transformed into a primarily anti-submarine force, hunting for Soviet submarines and mostly active in the GIUK gap. Following the collapse of the Soviet Union, its focus returned to expeditionary operations.

The Royal Navy maintains a fleet of technologically sophisticated ships, submarines, and aircraft, including two aircraft carriers, four ballistic missile submarines (which maintain the nuclear deterrent), six nuclear fleet submarines, six guided missile destroyers, eight frigates, eight mine-countermeasure vessels and twenty-six patrol vessels. As of September 2025, there are 64 active and commissioned ships (including submarines as well as one historic ship, HMS Victory) in the Royal Navy, plus 10 ships of the Royal Fleet Auxiliary (RFA). There are also four Point-class sealift ships from the Merchant Navy available to the RFA under a private finance initiative, while the civilian Marine Services operate auxiliary vessels which further support the Royal Navy in various capacities. The RFA replenishes Royal Navy warships at sea and, as of 2024–25, provides the lead elements of the Royal Navy's amphibious warfare capabilities through its three Bay-class landing ship vessels. It also works as a force multiplier for the Royal Navy, often doing patrols that frigates used to do.

The Royal Navy is part of His Majesty's Naval Service, which also includes the Royal Marines and the Royal Fleet Auxiliary. The professional head of the Naval Service is the First Sea Lord who is an admiral and member of the Defence Council of the United Kingdom. The Defence Council delegates management of the Naval Service to the Admiralty Board, chaired by the secretary of state for defence. The Royal Navy operates from three bases in Britain where commissioned ships and submarines are based: Portsmouth, Clyde and Devonport, the last being the largest operational naval base in Western Europe, as well as two naval air stations, RNAS Yeovilton and RNAS Culdrose where maritime aircraft are based.

The Royal Navy stated its six major roles in umbrella terms in 2017 as:

The Royal Navy protects British interests at home and abroad, executing the foreign and defence policies of His Majesty's Government through the exercise of military effect, diplomatic activities and other activities in support of these objectives. It is also a key element of the British contribution to NATO, with a number of ships or aircraft allocated to NATO tasks at any time. In 2007 core capabilities were described as:

The English Royal Navy was formally founded in 1546 by Henry VIII, though the Kingdom of England had possessed less-organised naval forces for centuries prior to this.

The Royal Scots Navy (or Old Scots Navy) had its origins in the Middle Ages until its merger with the English Royal Navy per the Acts of Union 1707.