The national arboretum in Gloucestershire was described by the Heritage Lottery Fund (HLF) as one of the world's most spectacular tree "museums". Its £1.9m grant will help pay for a new interpretation centre. It was one of six grant winners.
Money will also go towards reinstating rare grassland in the grade I listed Downland landscape and a programme of volunteer training to help conserve a site with 16,000 trees and more than 3,000 taxa (species).
Environmentalist and presenter Chris Packham said: "This announcement is particularly heart warming set against the awful backdrop of news of our endangered trees. Westonbirt is the natural world's equivalent of an outdoor museum."
HLF chief executive Carole Souter said: "Eighteen years of National Lottery funding has enabled us to invest £5bn. Thanks to lottery players, we're celebrating this milestone with grants to projects that illustrate the breadth of our heritage."
Other winners included the magical Swiss Garden in Old Warden Park, Bedfordshire, and the Flodden Field battlefield in Northumbria. The former received £2.8m to fund apprenticeships and restoration of structures such as a grotto and thatched seat.
The HLF also awarded £192,100 to firm up a full grant application of almost £3m to conserve the Winter Garden at Avery Hill Park in Eltham, London.