The 29-year-old made a £2m move to Villa Park in 2012 and made 56 appearances under Paul Lambert, scoring two goals.
El Ahmadi began his career with local club FC Twente before moving to Rotterdam giants Feyenoord in 2008.
He told the Dutch club's official website: 'I've always said I wanted to return to Feyenoord some time.
"That time has come. In my first period at this club Feyenoord was growing and that growth has continued in recent years.
"I'm really looking forward to playing in a full De Kuip again and sharing my experience with the other Feyenoord players.
"I'm also relishing teaming up again with Fred Rutten (Feyenoord boss), who was very important for me at the start of my career at FC Twente."
The departure of the Morocco international, who represented the Netherlands at youth level, could hasten the arrival of Manchester United midfielder Tom Cleverley who has been linked with a move to Villa Park.