The Hollywood actor, adored by women the world over, has filmed scenes for the ITV1 period drama especially for this year's Text Santa appeal.
According to The Sun, George - who is said to be a fan of Downton - will be starring as an American guest who attends a wedding at the country pile.
We CANNOT WAIT to see this!
A source told the newspaper that the part came about after George struck up a close friendship with Downton Abbey star Hugh Bonneville.
The pair worked on The Monuments Of Men together and stayed in touch.
"This is the biggest moment in Downton history and shows it's now the biggest drama in the world," the insider added.
ITV has confirmed George's guest appearance.
Meanwhile, the fifth series of Downton Abbey will return to our screens on Saturday 21 September at 9pm.
The new series will kick off with a 90-minute special and it's soon clear the household is struggling to keep up with the times in 1924.
Last year's Text Santa charity appeal saw Cheryl Fernandez-Versini rock up to the Rovers Return in ITV1 soap Coronation Street.
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