Plant Heritage, the former National Council for the Conservation of Plants & Gardens, made the award for five collections.
The arboretum already held the National Collection of Japanese maple cultivars, but four new National Collections have been added. These include collections of limes, maples, walnuts and bladdernuts. Westonbirt is home to four champion limes and a 2,000-year-old lime coppice.
A representative for Plant Heritage said: "These awards recognise a first-class broad collection of all ages and known origins. But they also recognise the work that goes on behind the scenes to record, care for and propagate these plants for the future."