Nelson has joined Hoffenheim on a season-long loan.
The midfielder stated: "I came to TSG to not only gain the highest level of match practice and develop myself further. On the contrary, I am convinced that with my qualities I can help the club achieve its ambitious goals in three competitions."
TSG chief Alexander Rosen also said, "I am delighted that we have succeeded in winning over a young player with such extraordinary skills.
"Reiss Nelson will give us a lot more options with his versatility in the No8 position, as an outside winger or a hanging forward."