The independent agency won the business after competing against Havas Media and the Publicis Groupe-owned ZenithOptimedia.
Vizeum, the incumbent, has handled Carnival’s media business since 2008, but the Dentsu Aegis Network agency was not involved in the recent process.
The account includes £8 million-worth of media activity for P&O Cruises. The brand is due to begin a major marketing push for the 2015 launch of Britannia, the biggest cruise ship ever built for the British market.
Carnival, a British-American business, will celebrate the 175th anniversary of Cunard next year.
Angus Struthers, the marketing director at Cunard, said: "The7stars clearly impressed during the pitch and we’re excited to be working with them as we continue to build the Cunard brand and advertising presence."
Liam Mullins, the head of trading at the7stars, said: "This is an important win for the7stars, allowing the agency to venture into new sectors and implement our fresh approach."
P&O Ferries, which is a separate business, continues to work with Havas Media.