The acquisition would create the country’s biggest pub group by turnover, with more than 3,000 pubs.
Hudson Sandler’s remit is to lead a comms programme to "further build positive awareness of Greene King as an innovative and responsible business among key stakeholders".
The agency was appointed without a competitive pitch. Greene King will continue to work with Finsbury for its financial PR.
Mark Blythman, strategy and comms director at Greene King, said: "We were impressed by Hudson Sandler’s relevant experience and expertise, and its support will be invaluable as we look to communicate our growth story and engage with our audiences."
Andrew Hayes, CEO at Hudson Sandler, said: "We look forward to amplifying this highly successful brand and supporting its strategy to connect with key issues in the pub and brewery industry."
Greene King, whose pub and restaurant brands include Hungry Horse, RealPubs and Old English Inns, currently has a 1,900-strong estate. It also owns beer brands including Greene King IPA and Old Speckled Hen.
Last year the group announced plans to buy Spirit, the Chef & Brewer and Flaming Grill operator that has around 1,200 pubs. In May, Greene King moved closer to its acquisition after offering to sell 16 pubs to satisfy the Competition and Markets Authority.
Separately, like other larger pub operators, Greene King is facing regulatory intervention under a new law to force pub groups to offer tenants the option of taking their pubs at market-only rents.