He made 26 appearances as he helped Bochum win Bundesliga 2 and promotion to the German top flight in 2021.
"He is a player who fits our profile very well and has a huge amount of potential," said Southampton manager Ralph Hasenhuttl.
"But, with the number of senior games he has already played, he can help us immediately.
"I think he has made the perfect choice in wanting to join us and we will now work with him to make sure he reaches his potential both for himself and for the team."
The Germany Under-20s international made 22 appearances last season as Bochum secured their Bundesliga status by finishing 13th.