The sponsorship, negotiated by Vizeum, includes broadcast, online and mobile. The idents will be produced by Walker Agency and use the strapline: "Discovering a different world, I can't wait ..."
Previously, the Downton Abbey sponsorship was included as part of the ITV Drama Premieres package sponsored by Aviva between 2009 and 2011. ITV is yet to announce a new partner for the rest of its drama slate.
Last year's series of Downton Abbey was the most-watched drama of 2011 with an average audience of 11.8 million viewers. The Christmas special was the most-watched show on Christmas Day. The new episodes will be screened this autumn.
Abbott Mead Vickers BBDO's idents for Aviva were toned down after viewers complained that their serious nature did not fit the tone of the programme.
Carol Marlow, the managing director of P&O Cruises, said: "We know many of our current and potential passengers watch Downton Abbey and we are excited to bring our brand to life for this audience."Follow @MaisieMcCabe