Karius was a free agent after his contract with Liverpool expired this summer.
The 29-year-old comes in as a short-term cover for Newcastle's United's back up goalkeeper Karl Darlow, who recently suffered an injury in training.
Nick Pope is firmly established as the club's number one goalkeeper following his summer switch from Burnley and Martin Dubravka has been allowed to join Manchester United on loan for the rest of the season.
Karius spent the last three seasons on loan at Besiktas and Union Berlin after losing his place at Liverpool to Alisson Becker.
Karius joined Liverpool from German side Mainz 05 in the summer of 2016 but he failed to impress when given the chance in goal and reserved arguably his worst performance in a Reds shirt for the 2018 UEFA Champions League final in Kiev when he was at fault for two of Real Madrid’s three goals.
Newcastle head coach Eddie Howe told the club website: "We are pleased to add Loris to our goalkeeping group.
"He is a very good goalkeeper with Premier League and European experience and he will provide competition and support over an important part of the season."
"I'm excited," Karius said upon signing a deal until January, which could be extended to the end of the season.
"It's a good opportunity for me and a great project to be part of. It didn't take me long to make a decision.
"Newcastle have a great coach and play really attractive football. I felt it was the right project for me and I'm excited to work with the coaches and my new team-mates."
