Trett was part of Grontmij and had a turnover of £14m last year.
The firm specialises in dispute management and resolution and operates out of UK offices in London, Leeds and Manchester.
The deal will see 75 Trett staff worldwide transfer to Driver.
Dave Webster, Chief Executive Officer of Driver Group said; “This acquisition provides a global platform for the full service offering of the Group plus opportunities to leverage our Project Services business – that Trett does not currently provide, across a world market and a broader sector and client base.
“The combined strength of two of the strongest brands in the dispute and advisory market will ensure both Trett and Driver clients receive unparalleled expertise and advice.
“As our businesses and culture are very similar the integration process should be reasonably straight forward and I am looking forward to working with Trett’s people in what for them is a return to an environment in which they belong.”
John Chubb, Managing Director of Grontmij in the UK and Ireland said; “Following the release of our ‘Back on Track’ strategy early this year, the disposal of Trett is an early move in the restructuring workstream as we focus on our core markets and core capabilities.
“I believe that the acquisition of Trett by Driver is an excellent fit which represents good value to both companies”.