The 2,500 sq m (26,909 sq ft) enclosure was designed and built by Arboretum staff and volunteers, and encircles a grove of oaks and other trees in which the squirrels live natural lives.
Ben Paterson, from the Castle Howard Arboretum Trust, said: “I can’t think of a better way to celebrate the start of autumn than by welcoming a new friend for David to the enclosure, she will bring much joy to our staff, volunteers and visitors.
“Cinnamon has arrived just in time for Red Squirrel Awareness Week, dedicated to increasing awareness of and action for the UK’s endangered red squirrels and their conservation, organised by the UK Squirrel Accord.
“We hope all our visitors that come to meet Cinnamon and David this month will learn something about the squirrels, delight in their playful behaviour, and feel inspired to support us in continuing to protect this lovely native animal.”
Yorkshire has only one colony of wild red squirrels, which live in a conifer plantation near Hawes in the Yorkshire Dales National Park. The land is managed to protect them.
The species is native to the UK but has suffered due to the loss of its preferred pine forest habitat and competition from American grey squirrels.