The original cast will reunite for the film and series' creator Julian Fellowes has already penned the screenplay.
Fellowes will also co-produce the film, which is being directed by Brian Percival, who directed the Downton pilot. Plot details are being kept under wraps.
Downton ran for six seasons on ITV in the United Kingdom and on PBS in the United States.
They all lived together in an Edwardian English country home at the turn of the 20th Century.
Yes, it's happening - the long-awaited Downton Abbey movie was given a green light on July 13, according to Variety. It starred Maggie Smith, Michelle Dockery, Brendan Coyle, Joanne Froggatt, Elizabeth McGovern, Hugh Bonneville, Jim Carter, Penelope Wilton, Laura Carmichael and Allen Leech.
Producer Gareth Neame said it had been "our dream to bring the millions of global fans a movie". Over its 6 seasons, the series garnered 3 Golden Globe Awards, 15 Primetime Emmy Awards, a Special BAFTA award and 69 Emmy nominations in total, making Downton Abbey the most nominated non-US television show in the history of the Emmys - even earning a Guinness World Record for the highest critically rated TV show along the way. The show was showered with awards, from Emmys to BAFTAs to Golden Globes - some more deserved than others - but it was always a joy to watch, even when it was occasionally ridiculous (the less said about Patrick the mystery man in bandages or Matthew's incredible "I've been paralyzed but I can definitely suddenly stand and catch a falling woman" miracle the better). Most notably, it gave Dame Maggie Smith a second career wind, as she played the juicy role of the acid-tongued Dowager Countess, Violet Crawley.
"Since the series ended, fans of Downton have always been waiting for the Crawley family's next chapter", said Focus Features chairman Peter Kujawski.
The official Twitter account for the series broke the news over social media to "cordially invite" fans to see the film, which will be screened in cinemas.