Van den Berg's contract with the Championship side runs until next summer, but he may have already played his last game for the club.
The defensive midfielder was signed by former Royals manager Jaap Stam but has seen opportunities limited under current boss Paul Clement, with the Dutchman being frozen out of their pre-season fixtures.
According to Football London, Van den Berg said: "I am very proud to be playing for NEC. The club has put a lot of energy into getting me to this club. I really appreciate that.
"In addition, I was immediately interested because I feel that NEC are doing something very beautiful here."
NEC's technical director Remco Oversier added: "With Joey we get a player who with his winning mentality and experience can play a leadership role in our selection.
"He not only has experience of a lot of Eredivisie matches, but also played an important role at Reading."
Van den Berg scored twice in 68 appearances for the Royals in total, with 34 of those coming last season.