The popular period drama will be broadcast on UTV next year, following the season four Christmas special, which will air on Christmas Day.
The special will feature two veteran American actors - Shirley MacLaine, who will reprise her role as Martha Levinson, and Golden Globe-winner Paul Giamatti, who will join the cast as family matriarch Cora Crawley's playboy brother, Harold.
The fourth series, which was the highest rated drama on television this year with an average of 11.8 million viewers, concluded on Sunday evening.
Gareth Neame, managing director of Carnival Films, which produces the show, said: "We promise all the usual highs and lows, romance, drama and comedy played out by some of the most iconic characters on television."
He added: "All the actors and makers of the show continue to be humbled by the extraordinary audience response and want to take the show from strength to strength next year."
Steve November, ITV's director of drama commissioning, said: "We are enormously happy to have Downton Abbey on ITV and we are delighted to be announcing this new series."
Created by Julian Fellowes, Downton Abbey debuted in 2010 and has since been sold to over 220 territories and is viewed by an estimated global audience of 120 million people.