Japan-based engineering group Nippon Koei has acquired all the shares of UK-based architecture firm BDP for £102.2m.
The merged entity expects to provide a combination of technical skills in engineering, architecture and integrated design solutions throughout the international markets.
BDP chief executive John Mcmanus said: "The alliance with Nippon Koei heralds an exciting new era in BDP's evolution. Although separated geographically, our two firms share common values and have highly complementary strengths in terms of professional offer and sector expertise.
"By uniting the two firms, we are creating an organisation that genuinely has the potential to be greater than the sum of its parts.
"We believe that this strategic international alignment will deliver huge benefits to our clients and will offer exceptional prospects for our people to develop both professionally and personally."
The company currently has six studios in UK and offices in Ireland, the Netherlands, the Middle East, China and India.
With 950 employees, the firm last year recorded revenues of more than £81m.
Nippon Koei president Ryuichi Arimoto said: "As a leading architecture and design firm in the UK, BDP's participation in the NK group will mark a great turning point in both of our histories.
"We will join forces on a global basis and provide solutions together to the many urban development problems from both an engineering and an architectural perspective.
"The combination of NK's 70 years of experience in social development and infrastructure together with BDP's 55 years of experience in the education, health and commercial sectors, will create significant opportunity to add value for our clients and, I am confident, will lead the group to become the world's leading integrated architecture, engineering and design company for the next generation.
"I also believe that the synergies between us will not only advantage our projects but will be of enormous benefit to our staff from a human resources perspective."
Nippon Koei said that with the acquisition it intends to expand its customer base mainly in the Asian region and help BDP to grow within Japan.