The Royal Navy of the United Kingdom is the oldest of the British armed services (and is therefore known as the Senior Service). From the early 18th century to the middle of the 20th century, it was the largest and most powerful navy in the world, playing a key part in establishing the British Empire as the dominant power of the 19th and early 20th centuries. In World War II, the Royal Navy operated almost 900 ships. During the Cold War, it was transformed into a primarily anti-submarine force, hunting for Soviet submarines, mostly active in the GIUK gap. With the collapse of the Soviet Union, its role for the 21st century has returned to focus on global expeditionary operations.

The Royal Navy is the second-largest navy of the NATO allies, in terms of the combined displacement of its fleet, after the U.S. Navy. There are currently 91 commissioned ships in the Royal Navy, including aircraft carriers, nuclear submarines, mine counter-measures and patrol vessels. There are also the support vessels of the Royal Fleet Auxiliary. The Royal Navy's ability to project power globally is considered second only to the United States Navy. The Royal Navy also has the second largest carrier fleet in the world, with 2 carriers in service and 2 planned.

The Royal Navy is a constituent component of the Naval Service, which also comprises the Royal Marines, Royal Fleet Auxiliary, Royal Naval Reserve and Royal Marines Reserve. The Royal Navy numbers 37,500 people of whom approximately 6,000 are in the Royal Marines.