In the deal, La Caisse acquired the Class A common shares held by Canadian packaging firm Cascade.
Boralex has developed over 1.1GW of wind capacity across Canada, the US and France.
As part of the acquisition, Boralex and La Caisse "have agreed to explore partnership opportunities with respect to investing in future projects".
"To have la Caisse come in as principal shareholder reflects the trust it puts in the business model and know-how of Boralex," said Boralex CEO Patrick Lemaire.
La Caisse will add two directorts to the Boralex board. It is a major investor in North American renewables. In July 2014, it acquired a 24.7% stake in US developer Invenergy.
It also has stakes in some French offshore wind projects, including St Brieuc and the floating wind site of Leucate in partnershipn with Engie and EDP Renewables.