Nippon Koei has made a £268m cash offer for Hyder which has been accepted by the board.
The announcement comes just days after Hyder bosses accepted a bid from ARCADIS valuing the firm at £256m.
A statement from the Japanese firm said: “The board of Nippon Koei has a strategic vision to become a top tier global engineering consultancy.
“It believes that a combination of Nippon Koei’s business with that of Hyder represents the opportunity to accelerate significantly this intent by forming a group with a breadth of technical capabilities across a range of complementary geographies.
“Nippon Koei anticipates that Hyder will continue to operate as a standalone division of the combined business within an operating structure designed to exploit the growth opportunities for both businesses.
“As such, the offer is not predicated upon cost synergies.”
The surprise move is expected to now spark a bidding war for Hyder.
Noriaki Hirose, President of Nippon Koei, said: “I regret that the confidential nature of our interest has meant that we have not yet had the opportunity to meet many of the employees of Hyder and begin planning our combined journey together.
“We recognise the value of collaboration and so I look forward to addressing this as soon as possible.
“A key element of our due diligence has been understanding the depth of Hyder’s engineering capability and the skills of its people, and we have been most impressed with what we have learned.”
Ivor Catto, Chief Executive of Hyder, said: “The cash offer from Nippon Koei announced today represents a 30 pence premium to ARCADIS’ offer announced on 31 July 2014 and accordingly the Hyder Board is now recommending the cash offer from Nippon Koei.
“The Hyder Board considers that Nippon Koei’s cash offer substantially recognises Hyder’s growth prospects, and provides certainty, in cash, to our shareholders today.
“The merged group should also provide further opportunities for our highly valued employees and clients.”