Westonbirt, The National Arboretum is an arboretum in Gloucestershire, England, about 3 miles southwest of Tetbury. Managed by Forestry England, it is perhaps the most important and widely known arboretum in the United Kingdom.

Planted in the heyday of Victorian plant hunting in the mid-19th century as part of the Westonbirt House estate, the arboretum forms part of a site which is listed Grade I on the Register of Historic Parks and Gardens of special historic interest.

Westonbirt Arboretum comprises some 15,000 trees and shrubs, with 2,500 species of tree from all over the world, covering an area of approximately 600 acres. Its 17 miles of marked paths are popular with visitors, and provide access to a wide variety of rare plants. There are two main areas to explore. The Old Arboretum is a carefully designed landscape offering beautiful vistas, stately avenues, and a host of rare and exotic trees from across the globe dating back to the 1850s. Silk Wood is a very different experience: although it also contains many exotic plantings, at its heart is a traditional working woodland, dating back to the 13th century. Dogs are welcome in Silk Wood but not allowed in The Old Arboretum.

The arboretum is managed by Forestry England and Westonbirt Arboretum is supported by the Friends of Westonbirt Arboretum, a registered charity which formed in 1985.

Westonbirt Arboretum is in Gloucestershire on the A433 approximately 3 miles southwest of Tetbury.