The Twilight: Breaking Dawn star will reportedly appear in special Christmas episodes later this year.
Buring will play a feisty servant named Edna in the period drama, reports The Sunday Times.
"You couldn't get two projects more different. I was thrilled when the Downton offer came," she said.
Bad Girls actress Simone Lahbib has also joined the cast as another servant.
The upcoming third series of the popular drama will begin in September, and will see the first appearance from Hollywood actress Shirley MacLaine.
Julian Fellowes has revealed details of the main plots of the third series.
