Royal Air Force Northolt or more simply RAF Northolt (IATA: NHT, ICAO: EGWU) is a Royal Air Force station in South Ruislip, 2 nautical miles (3.7 km; 2.3 mi) from Uxbridge in the London Borough of Hillingdon, western Greater London, England, approximately 6 mi (10 km) north of Heathrow Airport. As London VIP Airport, the station handles many private civil flights (private planes of up to 29 passengers) in addition to Air Force flights.

Northolt has one runway in operation, spanning 1,687 m × 46 m (5,535 ft × 151 ft), with a grooved asphalt surface. This airport is used for government and private VIP transport to and from London.

Northolt predates the establishment of the Royal Air Force by almost three years, having opened in May 1915, making it the oldest RAF base. Originally established for the Royal Flying Corps, it has the longest history of continuous use of any RAF airfield. Before the outbreak of the Second World War, the station was the first to take delivery of the Hawker Hurricane. The station played a key role during the Battle of Britain, when fighters from several of its units, including No. 303 Polish Fighter Squadron, engaged enemy aircraft as part of the defence of London. It became the first base to have squadrons operating Supermarine Spitfire aircraft within German airspace.

During the construction of Heathrow Airport, Northolt was used for commercial civil flights, becoming the busiest airport in Europe for a time and a major base for British European Airways. More recently the station has become the hub of British military flying operations in the London area. Northolt has been extensively redeveloped since 2006 to accommodate these changes, becoming home to the British Forces Post Office, which moved to a newly constructed headquarters and sorting office on the site. Units currently based at RAF Northolt are No. 32 (The Royal) Squadron, the King's Colour Squadron, 600 (City of London) Squadron, No 1 Aeronautical Information Documents Unit, the Air Historical Branch and the Central Band of the RAF.

Following Louis Blériot's first flight across the English Channel in 1909, the British Army considered the necessity of defending the United Kingdom from a future air attack. By May 1910, Claude Grahame-White and other aviation pioneers were flying from the flat areas around Ruislip, although they soon sought an aerodrome for London, which was eventually built at Hendon. A proposal was made in 1912 for the area around where RAF Northolt now stands to be developed as "Harrow Aerodrome". The company established to develop the site was listed on the London Stock Exchange but the idea did not progress any further.

The outbreak of the First World War necessitated a new aerodrome for the Royal Flying Corps. The Corps had received the Royal Warrant on 13 April 1912, whereupon Major Sefton Brancker of the War Office conducted aerial surveys in 1914 of Glebe Farm in Ickenham, and Hundred Acres Farm and Down Barnes Farm in Ruislip, looking for the most effective operating base for new squadrons. He settled on a site near Northolt Junction railway station; in January 1915 the government requisitioned the land. It is rumoured that the government official tasked with acquiring the land arrived at the site with his map upside down, leading to the government requisitioning and developing land on the wrong side of the railway line, including the old Hill Farm.

Construction of the new aerodrome, to be named "RFC Military School, Ruislip", began in January 1915. It opened on 3 May 1915, becoming known as Northolt and home to No. 4 Reserve Aeroplane Squadron which relocated from Farnborough. Most early RAF airfields were named after the nearest railway station, in this case Northolt Junction, later named Northolt Halt and now South Ruislip station; so the airfield became "Northolt" despite being in neighbouring South Ruislip. In the same year the airfield was extended westwards, and aircraft began flying sorties in defence of London against Zeppelin raids. No. 18 Squadron was formed in the same month as Northolt and equipped with Bleriot Experimental biplanes, whose slow speed led to heavy losses in combat with the German Fliegertruppe.

In 1916, No. 43 Squadron was formed under the command of Major Sholto Douglas. Aircraft equipping the squadron included the Sopwith 1½ Strutter, built by the Fairey Aviation company, then in Hayes. The Strutter made its first test flight from Northolt in 1916 with Harry Hawker at the controls. Fairey conducted test flights at Northolt from 1917 until 1928 when the Air Ministry gave the company notice to vacate the aerodrome. Flights later resumed from the Great West Aerodrome owned by Fairey in Harmondsworth, which was eventually developed as Heathrow Airport. No. 43 Squadron went on to fly sorties over France from 17 January 1917, taking part in the Battle of Vimy Ridge between 4 and 8 April 1917.