Gratterpalm, which has been briefed to develop a campaign for 2010, replaces Propaganda, which had held the business since 2006. Greggs' current ad campaign stars comedian Paddy McGuinness.
Greggs said the appointment was intended to integrate its communications strategy; it has worked with Gratterpalm on in-store design projects since 2008.
In October, the bakery chain announced plans to add a further 600 shops to its 1400-strong network of outlets. In August, Greggs posted a pre-tax profit of £16.5m for the first six months of the year, an increase of 7.3%.