Karius said he was "excited" about joining Newcastle
The 29-year-old was a free agent after leaving Liverpool in the summer and there is an option to extend the deal until the end of the season.
Karius has arrived after back-up keeper Karl Darlow suffered an ankle injury.
"I felt it was the right project for me and I'm excited to work with the coaches and my new team-mates," he said.
Newcastle lost fellow stopper Martin Dubravka during the summer when he joined Manchester United on a season-long loan, while Nick Pope joined from Burnley.
"He [Karius] is a very good keeper with Premier League and European experience," said boss Eddie Howe.
"He will provide competition and support over an important part of the season."
Karius played for Liverpool in their 2018 Champions League final defeat by Real Madrid and was blamed for two of the three goals - although it later emerged he had suffered a concussion.
He then had loan spells at Besiktas and Union Berlin before his Reds exit.
"I'm excited. 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," said Karius, who had joined Liverpool from Mainz 05 in 2016.
"Newcastle have a great coach and play really attractive football."
-- Courtesy of BBC Sport