The value of the transaction remains undisclosed, but EDF said the project would need £1.8 billion (€2 billion) in total investment to deliver the project, with commissioning planned for 2023.
EDF has a small footprint in offshore wind to-date. It owns the Blyth test facility in northeast England, and is part of a consortium struggling to develop 1.4GW of offshore capacity in France.
Last month, it was also revealed the firm was in talks to buy the stalled 24MW Atlantic City project in the US from Fishermen's Energy.
"This large-scale new offshore project demonstrates our strong ambition in being a leading global player in the offshore wind industry," said Bruno Bensasson, chief executive officer of EDF Energies Nouvelles, the French group's renewable energy arm.
"It confirms EDF Group's wider commitment to renewables in countries where EDF already has a strong footprint such as the UK. The project is consistent with the CAP 2030 strategy that aims at doubling EDF's renewable energy generation by 2030," Bensasson added.
Neart Na Gaoithe, set for a site off Scotland's east coast, received a Contract for Difference subsidy deal from the UK government in 2015 at £114.39/MWh (€140/MWh), in 2012 prices.
It was mired in a long-running court battle with UK charity the Royal Society of the Protection of Birds (RSPB), which said the Scottish government had failed to properly consider the environmental impact of the project.
This court delay meant the site almost lost its CfD deal after missing some contractual milestones. However, an arbitration tribunal backed Mainstream in March 2017, keeping the subsidy deal in place.
In January 2016, Mainstream announced it was in discussions with a consortium led by power company InterGen, to take the project to financial close and into construction. The consortium also included Siemens Project Ventures, the Marguerite Fund and Infrared Capital.
The site was set to deploy Siemens Gamesa Renewable Energy's 7MW turbine, following a deal between Mainstream and the manufacturer in 2014.
Siemens Group is also set to deliver its offshore transormer module (OTM) to the project. The OTM removes the need for a topside platform and can be installed across two turbine foundations. Mainstream said in 2015 it would save 30% in Capex costs.
Mainstream's chief executive officer, Andy Kinsella, said: "We are very pleased to be bringing in such an established partner and supporter of the Scottish energy industry in EDF Renewables to this vital infrastructure project for Scotland.
"The Neart na Gaoithe offshore wind farm has been fully developed by Mainstream and we are delighted to be handing over this world-class project."