The Morocco midfielder, 29, will move back to De Kuip on a three-year deal after two seasons at Villa Park as Tom Cleverley continues to be linked with a move to the Midlands club
The midfielder signed for the Villans from the Dutch club in 2012 and made 51 league appearances over two seasons in the Midlands, but has not played a game during the 2014-15 campaign so far.
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Paul Lambert also moved to bolster his midfield by signing Carlos Sanchez from Spanish side Elche earlier this month.
"I always said I would return to Feyenoord and now the time has come," El Ahmadi told the Eredivisie side's official website.
"During my first spell at this club, Feyenoord was growing and that growth has been continued in recent years. I'm really looking forward to getting back into a full Kuip to play and bring my experience to the team's other players.
"It is also a good prospect to be working with [head coach] Fred Rutten, who has been very important for me back at the beginning of my career at FC Twente."
El Ahmadi joined Feyenoord from Twente in 2008 and earned a move to Villa on the back of his impressive form under now-Southampton coach Ronald Koeman.
"Karim El Ahmadi wanted to return to the Netherlands," technical director Martin van Geel added.
"Moreover, El Ahmadi has already successfully worked previously with Fred Rutten and he is also familiar with playing for Feyenoord at De Kuip."