PNE formerly held an 89.1% stake in the German development firm, after initially acquiring an 83% stake in 2013, the parent company said.
The financial details were not disclosed.
PNE CEO Markus Lesser said: "Increasing our stake to 100% is another milestone in the implementation of our scale-up strategy.
"This will enable us to further optimise workflows and processes, and additionally strengthen the existing operational integration of the two companies.
"This way, we can realise additional synergies and increase productivity," Lesser added.
In its 2017 annual results released in March, PNE said it planned to expand into solar PV, storage and power-to-gas with a focus on hydrogen.
As part of a rebranding effort in early June, PNE said its new strategy also includes working across "the entire value chain of renewable energies as well as the refining of electricity".
WKN has developed almost 1.5GW of operating wind projects in France, Sweden, the US and Germany.