Brunswick has scooped the account after a competitive pitch, having worked with the group on projects in the past.
Nationwide announced it was merging with the Portman Building Society last year, finalising the deal in June this year by creating a £150bn mutual body.
Nationwide head of external affairs Alan Oliver said Brunswick would be offering strategic advice and corporate PR.
He added: ‘Recently all our support has been project-based. But post-merger we felt the need for continued support to maintain our reputation as a good press office.’
In the past few years Nationwide has performed well in an annual Ipsos Mori poll detailing which mortgage lenders and personal finance houses are the most trusted in the country.
The building society continues to use Hanover to boost the profile of its commercial lending arm, having brought the firm in earlier this year (PRWeek, 18 May).