The deal is worth €100m (£87m).
Marubeni has agreed to pay the cash in return for a 25% stake in Mainstream Renewable Power.
Last month, Mainstream, which is based in Ireland, submitted a £1.4bn application to construct a wind farm off the coast of Fife.
It said the facility would be ready by 2016 and could power 325,000 homes.
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Hundreds of jobs, both direct and indirect, could be created by the project, during the construction phase and its anticipated 25 years in operation.
The offshore project, called Neart na Gaoithe (which translates as 'strength of the wind' in Gaelic), would occupy an area of around 65 sq miles.
CEO of Mainstream, Eddie O'Connor, said: "This investment is a game-changer for Mainstream allowing us to focus on accelerating our project portfolios across a range of markets as well as entering into new strategic jurisdictions which present strong value opportunities for our business.
"Having grown our company in the midst of the global financial crises, this type of long-term strategic investment is true testament to the strength and experience of our team as well as the quality and spread of our global project portfolio."
(JP/CD)
