The value of the deal has not been revealed, but Partner's turnover is believed to be in the region of £750,000.
Partner founder Jonathan Hawker will join FD's issues management practice as senior vice-president, with responsibility for the European litigation practice.
FD chief executive Charles Watson said client concerns around corporate reputation meant that litigation and crisis support was one of the firm's fastest growing practices, and Partner's portfolio fitted closely with its expansion plans.
'Jonathan (a former TV news reporter) is an expert in crisis management and one of the top litigation comms practitioners in the business,' added Watson.
Partner brings clients including Tate & Lyle and Capita. Its only other staffer, Susie Tydeman, will join FD's European issues management practice as a consultant. FD poached three Hill & Knowlton corporate and financial executives earlier this year (PRWeek, 18 March).