It will pay £220 million (EUR 266 million) for the holding in the 576MW project which is currently under construction.
The bank said that the money made from the project's electricity generation would be reinvested back into other UK renewables projects.
Shaun Kingsley, the bank's CEO, said that it was investing "on fully commercial terms to create a demonstration effect which others will follow" in order to "mobilise capital into the UK offshore wind industry".
He added: "Helping developers to refinance part of their investments in developed projects is essential to ensuring that the industry is able to deliver the next round of new projects."
A number of major UK offshore projects have been cancelled in recent months leading to a crisis of confidence for investment in the sector. Most recently, SSE announced plans to drop two offshore projects with a combined capacity of more than 1GW and limit its involvement in around 11GW of Round 3 developments.
Indeed RWE has itself caused consternation after downsizing the Triton Knoll offshore project and abandoning the 1.2GW Atlantic Array offshore wind project altogether.
RWE announced in February that around three quarter of the Siemens 3.6MW turbines have been installed at the Gwynt y Mor site between Anglesey and the Isle of Man. The wind farm is expected to be completed by the end of the year.
The bank also announced today that it would buy a 50% stake with Marubeni in the 210MW Westermost Rough offshore wind farm from its developer Dong.