The 22-year-old had a five-month loan spell with Augsburg last season, scoring five goals in 17 games.
The transfer fee was undisclosed for a player whose contract with Sunderland was due to run out at the end of the season.
Ji joined Sunderland from K-League side Chunnam Dragons in June 2011 and played 28 times for the 'Black Cats', scoring twice.
Augsburg are eighth with 24 points from 17 games.
(Writing by Ken Ferris; Editing by Peter Rutherford)
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