Japan Bank for International Cooperation will also provide up to $92m in funds to help finance Metito's expansion plans.
Japan's Mitsubishi Corp and Mitsubishi Heavy Industries have acquired a 38.4 percent stake in water and wastewater project developer Metito Holdings for an undisclosed sum, the United Arab Emirates-based firm said.
In addition, the Japan Bank for International Cooperation will provide up to $92m in funds to Metito to help finance its expansion plans.
Japanese firms' global network and expertise will help Metito develop its operations, said Metito managing director Rami Ghandour.
"We have outgrown the region and we need to position ourselves for the next stage of our growth," Ghandour said in a telephone interview.
"We have some very aggressive plans and we feel there is definite room in the global water market for us to be the world's leading emerging markets player."
While Metito could look to bid for projects in collaboration with its new stakeholders, Ghandour said that would only happen for large projects which Metito couldn't do on its own.
The new funding secured under the deal will be used to expand some of the firm's existing schemes, as well as for new schemes in China and African markets it is targeting, including Ghana and Rwanda, he added.
Metito currently has a portfolio of water and wastewater assets worth in excess of $1bn, according to the company.
The Japanese transaction provides Abu Dhabi-based private equity firm Gulf Capital with a partial exit from its 2006 investment, reducing its holding in Metito to 23.8 percent from 56 percent, Metito said.
World Bank unit International Finance Corp cut its 6 percent stake in Metito, originally bought in 2007, to 3 percent under the Mitsubishi deal.
While not disclosing how much it made from the sale, Gulf Capital chief executive Karim El Solh said the offloaded stake returned more than three times its initial investment. He added that Gulf Capital planned to keep its remaining stake.
"We've distributed nice proceeds to our investors but we feel the remaining stake will be worth much more down the line. What we will do with it, we shall see in a few years but, for now, we are enjoying the exciting growth ahead," Solh said in the same telephone interview.
The stake sale ends the possibility that shares in Metito could be offered to the public any time soon - the company had previously said it was interested in a flotation, with London, Singapore and Hong Kong said to be options.
"We have the JBIC funds so that should keep us going in terms of funding new projects," said Ghandour.
"We are a capital-intensive business so at some time in the future we will need to examine what our funding strategy is. Whether that is an IPO or something else we will have to decide."