The deal comes after months of speculation that the London-based City PR giant - which has 450 staff in 17 offices worldwide - has been up for sale.
FD group chief executive Charles Watson said said: 'Joining forces with a recognised leader in the consulting world marks a landmark for the comms industry - creating the optimal environment in which FD will continue to thrive and grow.'
FD boasts more than 750 clients. It expects to exceed £66m in revenue for 2006.
The agency is currently owned by Advent International and FD's management.
FTI intends to operate FD as a separate business, run by its existing management team, including Watson and US CEO Declan Kelly.
For full coverage of the acquisition read PRWeek this Friday (15 September).