The German, a free agent following his departure from Liverpool in June, has agreed a deal until January with an option to extend his stay until the end of the season.
The 29-year-old's arrival follows an ankle injury sustained by Darlow in training.
"We are pleased to add Loris to our goalkeeping group," said Newcastle boss Eddie Howe. "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."
Karius was previously Liverpool's first-choice goalkeeper under Jürgen Klopp but never played for the club again after he made calamitous errors for two goals in the 3-1 defeat by Real Madrid in the 2018 Champions League final.
The goalkeeper said he was excited to make the move to St James' Park.
"Newcastle have a great coach and play really attractive football," he said. "I felt it was the right project for me and I"m excited to work with the coaches and my new teammates."