The new deal will keep Wendt at the Bundesliga club until the summer of 2018.
“Over the past few years Oscar has developed into a regular starter and a top performer,” said Gladbach sporting director Max Eberl.
“We are happy that we were able to secure his services with us for another two years. He will be particularly important for the younger players this season because of his international experience.”
“It is difficult to find another club in European football which is at the same level as Borussia,” said Wendt.
“The team, the fans, the stadium, they are all amazing. Moving to Borussia was one of the best decisions of my life and I am very happy to be able to continue playing here.”
Wendt joined Gladbach on a free transfer from FC Copenhagen in the summer of 2011.
— Borussia (@borussia) 17. august 2015