The 20-year-old Swede joins the Eredivisie giants on a season-long loan. Feyenoord have negotiated an option to buy the centre-half in the loan deal.
Jeng joined Reims, who were relegated from Ligue 1 last season, in January 2025 from Swedish side Sirius. The left-footed defender played just 120 minutes for Reims across all competitions last season.
"It feels good to be here, and I am proud to have signed with Feyenoord," said Jeng on the club website.
"Two days ago, I heard that the club were interested in me, and everything happened very quickly. It immediately felt like a good move. The stadium is fantastic, the club are top-notch, and I have only heard good things about the fans."
With Jeng, Feyenoord have completed a third summer move for a central defender, following the earlier signings of Anel Ahmedhodzic and Tsuyoshi Watanabe.
"Feyenoord are a big club that plays in a big league, so I'm looking forward to continuing to grow as a player here. The club have informed me that they see me as a talent, and I believe that Feyenoord have everything I need to develop my skills further. Hopefully, I can contribute to the club's success."