De Cat will sign a long-term contract believed to run until 2031, with the move completed for a fee reported to be around €23million including bonuses, plus a sell-on percentage retained by Anderlecht.
The transfer eclipses Hoffenheim's previous record incoming fee of €18million, paid to Bayer Leverkusen for Adam Hlozek two years ago. As De Cat only turns 18 on 19 July, the formal contract signing cannot be finalised until that date under FIFA regulations governing minors.
Hoffenheim's sporting director, Andreas Schicker, said: "We're delighted that a player as internationally sought-after as Nathan De Cat has made such a clear decision to move to TSG. That's an outstanding confirmation of the path we've taken.
"Nathan is one of the biggest talents in European football in his position. We firmly believe in his potential and want to give him the trust and the time he needs to show his qualities at Bundesliga level and to keep developing."
De Cat himself explained his reasoning behind the move: "TSG pursued me very early on and consistently. My conversations with the people responsible – Andreas Schicker, Paul Pajduch and coach Chris Ilzer – convinced me completely.
"TSG Hoffenheim offers me the ideal opportunity to prove myself in one of Europe's top five leagues and to contribute to the club's sporting success.
"On top of that, playing in the Europa League gives me the chance to gain valuable international experience too. TSG Hoffenheim has already shown time and again that it gives young players the best platform to become better players."
A product of Anderlecht's academy, De Cat made over 50 appearances for the Belgian club, scoring four goals and providing five assists, and earned his first senior international cap for Belgium in March. His rapid rise saw interest from Manchester City and Chelsea as well as several of Germany's leading clubs, but he opted for the platform Hoffenheim's project offered instead.
Hoffenheim's summer rebuild continues an active window that has already seen Cajetan Lenz arrive from Bochum, following the departure of Grischa Prömel to Stuttgart. With Europa League group-stage football secured for the coming season, Hoffenheim expect De Cat to get his first taste of continental competition at TSG.