The sale is part of SSE's disposal programme announced in March 2014 to reduce debt by £1 billion from the sale of non-core assets, said the utility firm.
Under the £355 million (€458 million) deal, Greencoat and the GMPF and LPFA Infrastructure (GLIL) pension fund joint venture acquired 49.9% of the 350MW operating part of the project. This share will then be diluted to 30% once the expansion is complete, SSE said. SSE will own 70% of the whole project.
Currently, Clyde's operating capacity is 350MW, but construction of a 172.8MW extension is ongoing, expected to be finished in June 2017.
The value of the deal is around £2.27 million per megawatt for the full project, once the extension is completed, SSE said.
The Clyde deal means SSE has exceeded the £1 billion debt disposal planand will "release capital to support future investment", the company said.
"The sale represents another significant step in a programme of disposals to recycle capital and optimise our wind farm pipeline. The proceeds from this disposal will help to support our future investments in a balanced range of energy assets," said SSE managing director for wholesale, Martin Pibworth.
GLIL comprises the Greater Manchester Pension Fund and the London Pensions Fund Authority. The joint venture is planning to invest £500 million in UK infrastructure projects in 2016 and 2017.