The BBC reported the third season will be set in the 1920s and consist of eight episodes. It is to premiere next year on British television, then subsequently on America's PBS stations.
Starring Hugh Bonneville and Maggie Smith, the show follows the lives of the wealthy Crawley family and their servants.
Series creator Julian Fellowes said he was "extremely grateful to ITV" for ordering a third season, the BBC reported.
"I have grown very fond of my Downton family and I certainly do not want to say goodbye to them quite yet," Fellowes said.