The Germany international is believed to have reached an agreement with Barcelona.
"I'm going to miss the stadium, the fans, the club, many people who are in the club, and of course my friends," Ter Stegen said after victory over Mainz.
"It was a very emotional game for me and I was delighted with the fans' support.
"I could say where I'm going whenever I want, but I always said I wanted to focus on the situation with Borussia.
"It was important to me that Borussia will be playing in Europe next week, and now we've done that."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball