The new brief brings together the UK press office account previously held by The Red Consultancy and the awards and reviews brief held by Kazoo.
Both had been held since 2009, and both agencies were invited to take part in a competitive process for the consolidated brief which began at the start of the year, run with help from intermediary The Observatory International.
Ketchum's work began on 1 July, led by client services director Susan Smith.
The Omnicom agency has also worked with Samsung on its Olympics sponsorship activation, with work lead from the tech firm's South Korean HQ.
Denise Kaufmann, CEO of Ketchum London, said: "We are thrilled to be working with Samsung. It is a market-leading, innovative tech company and we are looking forward to building on this great foundation to help them also become a loved brand. We will be using our integrated capabilities to focus on storytelling and content creation to drive the Do What You Can't sentiment and loyalty for the brand."
When Red won the account from Weber Shandwick in 2009, it was believed to be worth around £500,000 annually, but Samsung declined to comment on the value of the new brief.
As reported yesterday by PRWeek, Red has bounced back quickly from the loss, recently securing a UK and Western Europe brief with Huawei - an account overseen by a PR leader who recently moved to the Chinese company from Samsung.
Samsung continues to work with Taylor Herring on a project basis, and other marketing relationships are not affected.