The British Army is the principal land warfare force of the United Kingdom, British Overseas Territories and Crown Dependencies, a part of the British Armed Forces along with the Naval Service and the Royal Air Force. As of 1 January 2024, the British Army comprises 75,166 regular full-time personnel, 4,062 Gurkhas, 26,244 volunteer reserve personnel and 4,557 "other personnel", for a total of 110,029.

The British Army traces back to 1707 and the formation of the united Kingdom of Great Britain which joined the Kingdoms of England and Scotland into a single state and, with that, united the English Army and the Scots Army as the British Army. Members of the British Army swear allegiance to the monarch as their commander-in-chief, since both the English Bill of Rights of 1689 and Scottish Claim of Right Act 1689 require parliamentary consent for the Crown to maintain a peacetime standing army.

The army is administered by the Ministry of Defence and commanded by the Chief of the General Staff.

At its inception, being composed primarily of cavalry and infantry, the British Army was one of two Regular Forces within the British military (those parts of the British Armed Forces tasked with land warfare, as opposed to the naval forces), with the other having been the Ordnance Military Corps (made up of the Royal Artillery, Royal Engineers, and the Royal Sappers and Miners) of the Board of Ordnance, which along with the originally civilian Commissariat Department, stores and supply departments, as well as barracks and other departments, were absorbed into the British Army when the Board of Ordnance was abolished in 1855. Various other civilian departments of the board were absorbed into the War Office.

The British Army has seen action in major wars between the world's great powers, including the Seven Years' War, the American Revolutionary War, the Napoleonic Wars, the Crimean War and the First and Second World Wars. Britain's victories in most of these decisive wars allowed it to influence world events and establish itself as one of the world's leading military and economic powers. Since the end of the Cold War, the British Army has been deployed to a number of conflict zones, often as part of an expeditionary force, a coalition force or part of a United Nations peacekeeping operation.