Haileybury is an English co-educational public school (fee-charging boarding and day school for 11- to 18-year-olds) located in Hertford Heath, Hertfordshire. It is a member of the Rugby Group and enrols pupils at the 11+, 13+ and 16+ stages of education. Over 890 pupils attend Haileybury, of whom more than 550 board. The campus occupies over 500 acres of Hertfordshire countryside, approximately 20 miles from London.

Haileybury was judged 'Excellent in all areas' in its 2022 Inspection Report by the Independent Schools Inspectorate (ISI).

In 2022, 90% of A Level/IB grades were awarded at A*-B, or the equivalent.

In 2023, the school saw 43.9% of its candidates score A*/A

Haileybury hosts its own Model United Nations Conference every year, for over a thousand pupils, making it largest MUN conference in the UK. The conference is typically held the weekend before the Easter holiday.

The Haileybury campus originally belonged to, and was occupied by, the East India College, the training establishment founded in 1806 for administrators of the East India Company. The East India College was initially based at Hertford Castle, but substantial grounds in Hertford Heath were acquired for future development. William Wilkins, the architect of Downing College, Cambridge, and the National Gallery in London, was appointed principal architect. The buildings compose four ranges which enclose an area known as Quad, the second-largest academic quadrangle in Britain after Christ Church, Oxford.

The East India College closed in 1858 and, four years later, Haileybury College was set up as a boarding school for boys on the site. The first headmaster was Arthur Gray Butler. In 1942, Haileybury and the Imperial Service College (which had itself subsumed the United Services College) merged to become Haileybury and Imperial Service College, now referred to simply as Haileybury.

In the late 20th century, reforming headmaster David Jewell took charge of Haileybury, bringing it out of its post-Cold War austerity. Stuart Westley, Master of Haileybury until July 2009, was responsible for making the school fully co-educational.

In 2006/2007, Haileybury advised on the building of a Haileybury in Almaty, Kazakhstan where all English GCSEs are taught and the curriculum is taught similarly under the guidance of Haileybury. The school, opened in September 2008, is known as Haileybury Almaty.

The pupils are made up mostly of Kazakhstan citizens. They are all required to speak English. The academic year 2010–11 saw the first batch of pupils pass their IGCSE exams. Since August 2011, Haileybury Almaty has opened a sixth form. In 2016, 11 pupils graduated from the sixth form, with one getting admission to Trinity College, Cambridge University, and 6 securing places at University College, London (UCL). A second school, in the Kazakhstan capital, Nur-Sultan, was opened in September 2011.

Past pupils are known as Old Haileyburians.

For details of notable alumni, see List of people educated at Haileybury and Imperial Service College.

• Haileybury web site