The 27-year-old returns to Germany for the first time since joining the Reds from Mainz in 2016, having spent the last two seasons at Besiktas
The German shot-stopper, 27, has spent the last two seasons on loan at Besiktas having been ousted from the No.1 shirt at Anfield by Alisson.
Bundesliga minnows Union are looking to consolidate their place among the country’s elite after finishing 11th in 2019-20, their first-ever season in the top flight.
“I’m looking forward to my new opportunity in Berlin and I’m happy to be playing in the Bundesliga again,” Karius told Union’s official website.
“Union is a special club, one that has earned a great deal of respect, not just by promotion to the Bundesliga. I would like to play my part in this positive development and help to achieve the great goal of staying in the Bundesliga.”
Football director Oliver Ruhnert added: “The signing of Loris Karius means we have secured a goalkeeper with great national and international experience.
“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.”
With Alisson now firmly established as Jurgen Klopp’s No.1, it is perhaps unlikely that he will return to play a significant role at Liverpool, but he will no doubt be hopeful of rediscovering his confidence and form back in his home country.
Union’s next game is a home fixture against his former side Mainz.