The agency will be solely responsible for UK comms and will devise a strategy to be rolled out across Europe.
According to financial reports by owner Diageo, Baileys has undergone a ‘change in promotional strategy’.
The brand has repositioned itself towards women and younger audiences after disappointing sales in Europe.
Preliminary results to 30 June show there was a seven per cent sales drop for Baileys across Europe compared with the same period last year.
Senior PR manager Rachael Newton said: ‘The appointment follows the launch of the Baileys "Cream with Spirit" campaign to celebrate the spirit of modern womanhood.
‘Working closely with Baileys’ other media agencies, Unity will be focused on dialling up relevance and meaning for the brand among its target female audience.’
The brief does not include social media, which is carried out by digital agency Dare.
Ketchum lost the account after five years due to conflict issues. It will work with Unity until the end of the year. Unity declined to comment.