The actress, who has appeared in Doctor Who, will play the governess of heartbroken widowed chauffeur Tom Branson's (Allen Leech) daughter Sybil and helps him to get over his late wife, said the Daily Mirror.
A source said: "Daisy will become one of Downton's most prominent characters. She wowed producers in her audition and they are delighted to have secured such a brilliant young actress.
Daisy Lewis has reportedly been cast in Downton Abbey
"Her romance with Tom will be one of the main storylines of the series. He will begin to develop feelings for her and they start a secret affair. Tom will have an even bigger role in the new series after becoming the estate manager of Downton. The writers know he is a very popular character and the viewers were gripped by Lady Sybil's traumatic death."
Daisy is one of a host of new characters joining the show, including Paul Giamatti as Lady Cora's playboy brother, opera singer Dame Kiri Te Kanawa as a house guest, former EastEnder Nigel Harman as a visiting valet and Death In Paradise star Gary Carr as a jazz singer.
Branson and Lady Sybil
Joanna Davis, Julian Ovenden, Tom Cullen and Dame Harriet Walters have also been added to the line-up.
The new series of Downton Abbey is set to air this autumn on ITV.