The drama, which focuses of the lives of the aristocratic Crawley family and their servants, will go into production next year.
This is in addition to the special one-off episode which will air at Christmas.
Created by writer Julian Fellowes, who is set to return for the next series, the show first aired in 2010.
"It is fantastic that Downton continues to be such a phenomenon - still the most popular drama on ITV in its fifth series - and we are thrilled to have commissioned a sixth series," said Steve November, ITV's director of drama commissioning.
"We don't know yet what Julian has planned, but we are looking forward to working with him, the fantastic cast and Carnival again, and have no doubt series six will be unmissable."
Producers have yet to confirm the full cast.
Earlier this week Hugh Bonneville, who plays Lord Grantham, talked to Newsbeat about some of the celebrities, including Kim Kardasian and Kanye West, who are said to be fans of the show.
"It's only a matter of time before they are on the show," joked Bonneville about the pair.
"Hillary Clinton and John Kerry [United States Secretary of State] are fans of the show too.
"To have them like the show as well as trying to run countries and economies is great."
The fifth series of Downton Abbey ends on Sunday 9 November.
Follow @BBCNewsbeat on Twitter and Radio1Newsbeat on YouTube
Downton Abbey will be coming back for a sixth series in 2015, ITV has confirmed.