Berghuis scored seven goals and recorded six assists in 30 appearances for Feyenoord last season as they pipped Ajax to the league title by a single point on the last day of the campaign.
The Netherlands international made only nine appearances for Watford in his two years at the club, with Feyenoord paying a reported £5.8 million to sign the 25-year-old.
"Steven proved last season that he is a player with exceptional qualities," said Feyenoord's technical director Martin van Geel. "With his creativity, free-kicks and nose for goal he was very valuable to us.
"So I'm happy that we have reached agreement with Watford and Steven and that we have been able to contract him before the first official match of the season."
Feyenoord get the new campaign under way on Saturday when they face Vitesse Arnhem in the Super Cup, before hosting Twente in their first league game on August 13.