The Dutch-born Moroccan international played for Feyenoord between 2008 and 2012, during which he had a loan spell in the United Arab Emirates, before moving to England.
At Feyenoord, El Ahmadi reunites with coach Fred Rutten under whom he played in the early stages of his professional career at Twente Enschede.
"I always said I'd like to come back to Feyenoord and the right moment has arrived," El Ahmadi told the club website (www.feyenoord.nl).
"In my first spell, Feyenoord were a growing club and have continued this over the last years. I'm looking forward to playing in front of a capacity stadium at De Kuip and helping the other Feyenoord players with my experience," added the 29-year-old.