Impax has sold 11 projects in France totalling 124MW and six in Germany adding up to 82MW. The transaction's enterprise value amounts to around €297 million.
The deal with ERG Renew also includes the sale of two companies - one in Germany and France - that operate and commercially assist the projects. It is expected to be closed later this year.
Impax said the sale was part of its strategy, having owned and grown the portfolio through acquisitions from 2010 to 2014.
The asset firm said it would retain a wind project development company in France that it will continue to "expand significantly through construction and acquisition in the coming years".
Impax managing director Daniel von Preyss said: "[This deal] demonstrates the success of our investment strategy of consolidating our European renewable energy assets into strategic portfolios for exit, as we seek to provide strong returns to our investors."
ERG Renew owns just over 1GW of operating capacity in Italy, with a 13% share of the market. Across Europe, ERG said it operates 1.7GW of wind.
Luca Bettonte, ERG chief executive officer said: "The transaction, which allows the Group to position itself as ninth onshore wind power operator in France and eighth at European level, involves two countries where renewables are considered an essential factor of their energy policy and which we therefore view as fundamental for our future growth."
The deal with Impax doubles ERG's owned capacity in France to 251MW.