English: The Alnwick Garden in England is a complex of formal gardens adjacent to Alnwick Castle in the town of Alnwick, Northumberland, England. The gardens have a long history under the Dukes of Northumberland, but fell into disrepair until revived at the turn of the 21st century. The garden now features various themed plantings designed around a central water cascade.

Redevelopment of the garden was instigated by Jane Percy, Duchess of Northumberland in 1997, and has been led by Belgian landscape designers Jacques and Peter Wirtz. It is the most ambitious new garden created in the United Kingdom since the Second World War, with a reported total development cost of £42 million.

The first phase of redevelopment, opened in October 2001, involved the creation of the cascade and initial planting of the gardens. In 2004 a large 6,000 sq ft (560 m2) tree house complex, including a cafe, was opened. It is one of the largest treehouses in the world. A pavilion and visitor centre designed by Sir Michael Hopkins and Buro Happold opened in May 2006, with capacity for 1,000 people. The pavilion and visitor centre feature a barrel-vaulted gridshell roof. The gardens include several water features as well as architectural landscaping, topiary and decorative gates.

A garden featuring intoxicating and poisonous plants was added in February 2005. Species of the Poison Garden include Strychnos nux-vomica (source of strychnine), hemlock, Ricinus communis (source of harmless castor oil but also deadly ricin), foxglove, Atropa belladonna, Brugmansia and Laburnum. The mission of the Poison Garden also includes drug education, with featured plantings of cannabis, coca and the opium poppy Papaver somniferum