Dare won the business after a pitch against Draft FCB and Leo Burnett, which kicked-off in June last year.
The previous incumbent Mother resigned the business mid-way through the pitch process, citing "internal conflict" at the Post Office as a factor behind the decision.
Dare, which relaunched this month following its merger with sister Cossette agency MCBD, will hold the contract for a three-year period, and will be responsible for handling all of the Post Office's TV advertising, as well as the creative development and production of its radio, outdoor, press and digital work.
Martin Moran, the newly appointed commercial and marketing director at the Post Office, said: "We were very impressed by the quality of all the pitches we received and want to thank all of the agencies involved. We look forward to working with Dare to develop engaging campaigns promoting our competitive range of products and services which underline the Post Office’s unique role in communities across the UK."
Mother first won the advertising for the Post Office in March 2007, following a pitch against Grey London and DDB UK.