The Germany international is widely expected to join Barcelona in the close-season, with the Catalan club chasing a replacement for the outgoing Victor Valdes.
And Gladbach sporting director Max Eberl has confirmed Ter Stegen will leave Borussia-Park after the current campaign's conclusion, though remained coy on his destination.
"After Marc-Andre ter Stegen has informed us that he definitely wants to leave at the end of the season, we have now reached an agreement with his new club on the transfer arrangements," Eberl told the club's official website.
"He will move abroad, that's all for now.
"His successor in goal for Borussia will be Yann Sommer.
"He is an excellent goalkeeper who has already gained a lot of international experience in his 25 years and we are delighted that he has chosen to sign for us."
Sommer has five caps for the Swiss national team and is expected to be in their squad for this year's FIFA World Cup.
Having been Basel's first-choice keeper since 2010, Sommer has made 152 appearances, racking up three consecutive Swiss Super League titles.
