The agency was brought on board on the back of a successful one-off campaign it undertook for the company in December last year.
Big will handle the PR support for up to four UK-wide campaigns over the next year.
Big director Alex Barr will head up the account and report into Bauer's marketing director for radio charities Dawn Gibson.
The agency will work across Bauer Radio's 16 radio stations across Scotland and Northern England. Stations include Metro in Newcastle and Radio Forth in Edinburgh.
Bauer Publishing bought Emap's Radio division at the beginning of the year and rebranded the business to Bauer Radio.