The RSPB is to create its first nature reserve in the New Forest after buying 386 hectares of woodland.
The bird conservation charity said the "secret forest", near Nomansland on the Wiltshire-Hampshire border, was one of its "most significant" acquisitions.
It contains an "internationally important lichen community", as well as hawfinch, spotted flycatcher and wood warbler bird species.
The formerly privately-owned land will be renamed Franchises Lodge.
It was given to the nation in a tax settlement and was bought with the help of a £200,000 grant from the New Forest National Park Authority.
RSPB regional director Nick Bruce-White said: "We think it might hold some real wildlife treasures."
The charity said it would consult local residents before deciding on granting public access beyond the existing rights of way.