He is yet to make his first competitive appearance, but is expected to be Urs Fischer's first choice after the international break after making his debut against Hannover 96 in a friendly fixture.
“The signing of Loris Karius means we have secured a goalkeeper with great national and international experience," said Oliver Ruhnert, Union's director of football.
“His strengths are well known, so we are pleased to be in an excellent position in this important role of the team from our perspective. For Loris it offers the chance to focus again on the Bundesliga in a stable environment and to try to stay in the league with Union."
Manager Urs Fischer suitably impressed with his new recruit after their 4-1 victory over Hannover at the weekend.
“[The first thoughts are] positive – it wasn't easy for him," he said. “In the first half, he had almost nothing to do. A shot that he had to keep – otherwise he was unemployed."