Terms of the deal were not disclosed.
'Dittus has been on our radar for some time,' said FD's US chief executive Declan Kelly.
'We see a considerable convergence and growth in the international public affairs and regulatory environment,' he added.
Dittus's clients include Home Depot, Qwest, Tyco, Dell, SAP, and the Pharmaceutical Research and Manufacturers of America.
FD held a management summit in London last month to discuss how Dittus, LLM, and other public affairs offices would begin to share their knowledge.