The channel had kept its audience of almost 12 million waiting longer than usual to confirm a further run of the show, scripted by Julian Fellowes.
The eight-part run next year will open and close with feature-length episodes as the tales of the Crawley family and their servants in the early 1920s continue, with filming once more at Highclere Castle.
There will also be another Christmas edition.
This year's two-hour festive episode sees the family head to a Scottish estate – filmed at Inveraray Castle – to visit cousins, against a backdrop of deer stalking in the Highlands.
But many of their staff remain at Downton, and further dramas unfold in the family's absence.