The two-minute trailer will be shown on ITV1 after the special festive episode of the country house drama.
ITV's Rufus Radcliffe said: "Titanic promises to be a massive TV event in 2012, and what better moment to start building anticipation amongst our viewers than an epic trail coming out of ITV's Downton Abbey Christmas special."
The drama, which is made up of four 60-minute episodes, counts Linus Roache, Celia Imrie, Toby Jones, Geraldine Somerville, Sophie Winkleman and Timothy West among the cast.
It follows the story from the building of the Titanic at the Harland and Wolff shipyard in Belfast to its loss at sea after it hit an iceberg on its maiden voyage in 1912.
The set includes a 60-metre-long deck, four working lifeboats and 75 metres of corridors as well as cabins and a boiler room.
Director Jon Jones said: "It's an incredible, evocative playing space. We have built a huge section of the ship, that is jam packed with a warren of rooms arranged to allow us a sense that we have the run of the ship.
"Once you enter the studio, it is like boarding the ship - you get a real sense of the space. We want to get to grips with how it really was to be confined in a tiny cabin in steerage - or effortlessly ensconced in a First Class stateroom. Our set allows us to really enter that world."
The trailer can also be seen on the ITV player from noon tomorrow.
The series will be shown in April 2012 to mark the 100th anniversary of the ship's sinking.