Jo Murray has held the party's top communications role since 2005 and is responsible for creating and delivering national and regional as well as specialist media strategies.
She will move to Fishburn Hedges in March. Agency chief executive Simon Matthews said Murray's media relations and campaigning experience would be 'invaluable' to the agency.
Murray has held a number of press office roles at the Labour Party since 1998, including that of chief spokesperson. The party is currently recruiting for her successor.