The Swansea City midfielder was out of contract since May 2019.
He was linked with a move to Sheffield United in this transfer window.
However, Fer preferred to return home to Feyenoord.
He expressed his delight at the transfer being completed.
"My heart is here. I'm very happy to be back," Fer told the Feyenoord website.
"I feel stronger and fitter and I will do everything in my power to get back to my old level as soon as possible, in order to have a great season with Feyenoord."
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