The Kew Herbarium (herbarium code: K) is one of the world's largest and most historically significant herbaria, housed at the Royal Botanic Gardens, Kew in London, England. Established in the 1850s on the ground floor of Hunter House, it has grown to maintain approximately seven million preserved plant specimens, including 330,000 type specimens. The herbarium's collections, which include specimens dating back to 1700, represent about 95% of known vascular plant genera and 60% of described fungi, with specimens collected over 260 years of botanical exploration. The herbarium processes around 5,000 specimen loans annually and hosts approximately 3,000 visitor-days of research visits each year, supporting a wide range of botanical research.

The herbarium's development has been closely tied to British botanical exploration and colonial expansion, with contributions from influential directors like Sir Joseph Dalton Hooker and major acquisitions including the Gay Herbarium. Research at the herbarium has contributed to botanical taxonomy, with publications such as the Index Kewensis, initiated with funding from Charles Darwin in 1882, and the Kew Record. The facility has undergone several major expansions since its first purpose-built wing was constructed in 1877, with Victorian architecture that includes spiral staircases, iron columns, and vast handcrafted wooden cupboards. In 2022, the herbarium initiated a £29 million digitisation project to produce high-resolution images of its collection, with a target completion date of 2026.

The herbarium is a resource for botanical research in taxonomy, conservation, ecology, and climate science. Its specimens offer data that aid in tracking environmental changes, studying plant diseases, and identifying new species, as demonstrated by the 2022 discovery of Victoria boliviana, the world's largest water lily species, which had remained unrecognised in the collection for almost two centuries. The herbarium's future is currently subject to debate, with controversial plans announced in 2023 to relocate the collection to Thames Valley Science Park, prompting discussion about the balance between preservation needs, research accessibility, and maintaining the historic connection between the herbarium and Kew's living collections.

Prior to the establishment of Kew's herbarium, botanical specimens in London were primarily studied at Sir Joseph Banks' collection in Soho Square. This changed after William Jackson Hooker became Kew's Director in 1841, when he began allowing researchers to access his substantial private collection at his residence near the gardens. The herbarium officially began in 1852 when the ground floor of Hunter House, an early 18th-century Queen Anne style building on Kew Green, was allocated to store both Hooker's collection and the herbarium and library of William Arnold Bromfield. Hunter House had previously been the residence of the Duke of Cumberland (later King of Hanover) until his death in 1851, and formed part of a larger complex with the adjacent Meyer's House (now Hanover House). In 1876, parts of Hunter House, including its drawing room, south room, kitchen and river frontage steps, were demolished to make way for the herbarium's first purpose-built wing. The following year saw the appointment of the first curator, Allan Black.

The herbarium's collection includes specimens from many notable historical figures, illustrating its connection to exploration and scientific discovery. Among its treasures is a specimen of Faroa nyasica collected by David Livingstone in the 1860s during his explorations of what is now Malawi. While the collection includes a 4,000-year-old olive tree branch from Tutankhamun's tomb, its oldest officially catalogued specimen is Indigofera astragalina, collected by Daniel du Bois at Fort St. George in India in 1700. Other early specimens are those collected by Samuel Browne, a surgeon working for the East India Company in the 1690s. Browne collected plants around Madras (now Chennai) and collaborated with Tamil and Telugu speakers to record local plant names and uses.

The early herbarium grew rapidly through both government grants and private donations. In 1853, the herbarium received William Bromfield's Flora Vectensis. An expansion took place in 1854 when George Bentham donated his herbarium and library to the nation, on condition they would be housed at Kew and remain accessible to botanists. After William Hooker's death in 1865, the government purchased his herbarium and library in 1866.

Many significant historical collections were incorporated during this early period, including Allan Cunningham's Australasian specimens, William John Burchell's collections from Saint Helena and South Africa, Robert Brown's British specimens, Hewett Watson's British collections, Amelia Griffiths's algae, William Wright and Johan Peter Rottler's Indian specimens, and several general herbaria. The extensive Indian collections of Hooker and Thomas Thomson that reached Kew in 1851 contained an estimated 8,000 species. Another valuable acquisition was the herbarium of Jacques Gay, which arrived at Kew in 1868. This collection included early specimens from Senegal collected in the 1820s by Claude Richard, the founder of the Richard Tol botanical gardens, and by Döllinger (who collected in Senegal during 1823, particularly around Richard Tol). Many of these specimens were originally sent to Gay by Baron Jacques François Roger [fr], the governor of Senegal (1821–1826), who played an important role in furthering botanical research in the region.

The first purpose-built wing of the herbarium was constructed in 1877, prompted by the need for additional space to accommodate botanical specimens collected during exploration of the British Empire. Around 1878, a large hall was added – a quadrangular structure eighty-six feet by forty-three feet, with a ground floor and two galleries connected by two spiral staircases and lit by forty-eight windows. A second hall of the same dimensions was completed around 1903. The facility underwent several further expansions between 1902 and 1969, including basement extensions. The facility has expanded periodically to house its growing collections, with extensions added approximately every 40 years. This pattern of growth continued with a modern wing added in 2009.