With a combined workforce of more than 1,000 architects, the newly-formed 'super studio' will boast offices in 16 cities across America, Europe, Asia and the Middle East.
The move, which will see the US offices working under the new banner RMJM Hillier, is a welcome boost for RMJM which has been slowly slipping down the AJ100 list since its involvement in the Scottish Parliament debacle.
According to the latest figures, the practice could only muster 65 qualified architects in the UK - compared to 70 in 2005/6 - resulting in a fall down the chart to 31st place.
However, following the merger, the RMJM Group can claim responsibility for projects with a construction value totalling more than £7.5 billion.
'Never before in the history of architecture have clients had such a global depth of architectural talent to meet their design aspirations,' said Peter Morrison, chief executive of the RMJM Group.
'The new firm will draw on a skill set from our diverse and experienced professionals that is unmatched and can be perfectly tailored to meet the design needs of our client base.'
Based in Princeton, New Jersey, Hillier was set up in 1966 and has outposts in Philadelphia, New York and Washington DC, as well as Shanghai.
It is understood RMJM will continue working under its existing title in the UK.