The reserve is now large enough to support breeding hen harriers, with the "red-listed" bird seen occasionally in the area but not recently attempting to breed, the charity said.
It has been active at Gwenffrwd-Dinas for more than a century, first by supporting the Kite Committee in their efforts to protect red kites, and later by acquiring land to protect vulnerable habitats and wildlife.
With Gallt-y-bere now protected, RSPB Cymru said it was taking another important step towards creating a more resilient future for Wales' wildlife.
Surveys over the summer, funded by Natural Resources Wales, will help identify the full range of habitats and species present across the newly acquired land and inform future management.
Existing wildlife-rich grasslands may be expanded and enhanced, providing greater resilience for important plants and wildlife, including species such as whorled caraway and wood bitter-vetch, the RSPB said.
The charity added that it would also create opportunities to work closely with local farming tenants to demonstrate how nature recovery and sustainable farming can successfully work hand in hand across the wider landscape.
In the future, RSPB Cymru said it planned to work with local communities and partners to restore and protect the newly connected landscape, ensuring it supports wildlife for generations to come.
