The stake has been bought by the city of Bedburg, giving the project an element of community control.
It will be built on the site of the Garweiler open-cast mine near Bedburg, North Rhine-Westphalia, where RWE mines lignite. Construction work is expected to start in the coming weeks, RWE said.
The German utility will continue to hold a 51% stake in the project, which will be its largest onshore wind farm in Germany.
RWE, one of Germany's four largest energy companies, caused increasing consternation in the UK in recent months by selling stakes in several onshore and offshore wind farms.
The moves are part of a repositioning by the company to deal with problems that include drastically reduced earnings due to ongoing low wholesale electricity prices at a time when the company struggles with a massive EUR 30 billion in debts.