The company has hit the headlines several times this year after investigations were launched into the care offered in some of its facilities.
Porter Novelli has been called in to provide crisis support and to encourage the company to adopt a more proactive approach to PR.
Southern Cross initially had talks with 12 agencies before asking six to pitch. PN won the brief after a final two-way shoot-out.
PN's brief includes corporate, consumer and crisis management. The agency will also provide national press office support for all five regions of its business. PN will create a team of press officers spread across London, Banbury and Edinburgh.
The annual budget will be a six-figure sum and the agency takes over from incumbents Recognition PR and Media House International.
Southern Cross marketing manager Pauline Boyne said: 'We want to get more on the front foot with our marketing effort. As the largest care home provider in the UK, there are plenty of challenges in managing our reputation, but there's also strong potential to develop our brand in ways that can help drive our business forward. Choosing a new partner for PR is a positive step for us and Porter Novelli stood out with its strategic vision, strong creative ideas and all-round ability.'
The account will be headed up by PN head of crisis Neil Bayley.