The 20-year-old Stark, who is now an Under-21 international, trained with the squad after completing his medical.
"A young player who has great potential has joined us," said Hertha CEO Michael Preetz in a statement. "He is a midfield player who can also be deployed as a central defender. We are happy he decided in favour of our club."
The tall Stark captained Germany to the Under-19 European title last year. He has won three caps for the Under-21 team.
Hertha have got off to a good start in the Bundesliga with four points from their opening two matches and sit in seventh place.
(Reporting by Karolos Grohmann; Editing by Ken Ferris)
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