Ter Stegen left the field on a stretcher after getting injured in a 5-1 win over Villarreal in the Spanish league on Sunday.
Ter Stegen's right knee buckled when he fell awkwardly after going for a high cross into the area just before halftime.
Barcelona said that surgery on Ter Stegen's knee was planned Monday after tests confirmed "that he has a complete rupture in the patella tendon in his right knee."
The injury comes just as Ter Stegen had been confirmed as the German national team's No. 1 goalkeeper after years as Manuel Neuer's backup.
Neuer retired from international soccer after Euro 2024 and coach Julian Nagelsmann said Ter Stegen — who has played 42 games for Germany but none at a major tournament — would be first choice in the future. However, he only played two Nations League games before his injury.
Germany's most recent squad included backup goalkeepers Alexander Nübel from Stuttgart and Oliver Baumann from Hoffenheim. Neither has played a game for Germany.