The Green Investment Bank said it was buying a 50% stake in the Westermost Rough wind farm off the coast of East Yorkshire and a 10% stake in the Gwynt y Mor project off the coast of North Wales.
The Welsh wind farm is the biggest of its kind under construction in Europe and, once operational, will generate enough electricity for 400,000 households.
Westermost Rough, currently in the early stages of construction, will use turbines almost twice the size of most of those currently operating in UK waters, and will generate enough power for 200,000 homes.
Shaun Kingsbury, chief executive of the Green Investment Bank, said: "The UK has ambitious plans to build on its position as a world leader in offshore wind.
"We have two roles to play in supporting those plans: firstly, to directly invest to help developers recycle their capital into the next wave of new renewable energy projects; secondly, to invest on fully commercial terms to create a demonstration effect which others will follow."
Business Secretary Vince Cable said: "The Green Investment Bank is playing a game-changing role in financing our transition to a green economy.
"The bank has now invested well over £600 million in five offshore wind farms and mobilised £1.3 billion of total funding. This industry has the potential to generate thousands of new skilled jobs and billions in business investment."
Energy and Climate Change Secretary Ed Davey, said: "The UK is number one in the world for investment in offshore wind.
"This news is proof of the strength of investment in British energy infrastructure - and proof that the Government's strategy is working - supporting green jobs across the country and delivering secure energy supplies for the future.
"£34 billion worth of investment has been announced since 2010 thanks to our electricity market reforms which give investors long-term certainty."