Fallon picked up the business after a pitch, run through the AAR, against Creature and The Brooklyn Brothers, which kicked off in September last year.
The mobile brand split with Albion, its incumbent of three years, prior to calling the review. The shake-up came six months after the arrival of former Procter & Gamble marketer Emma Jenkins as chief marketing officer at Giffgaff.
Jenkins said: "It's fantastic to have Fallon working with us. We are looking forward to delivering creative campaigns in a Giffgaff way".
Fallon will start working with Giffgaff straight away.
Gail Gallie, chief executive of Fallon, said: "Giffgaff has a brilliantly innovative approach to their business, and has real ambition to challenge the category."
Giffgaff’s most recent advertising includes its sponsorship of E4’s ‘The Big Bang Theory’. The sponsorship idents asked consumers to suggest which items it should blow up, with one of the executions featuring an exploding cake.
Albion led brand development for the business since its launch, having been appointed by O2 in May 2009 after a pitch also via the AAR.
The agency’s remit included all advertising, social media, website design and film idents for the brand.