The 29-year-old ended his six-year spell with Liverpool, having spent last season entirely on the periphery after two different loan spells.
Karius did not feature for the Reds following the 2018 Champions League final, instead spending time with Besiktas and Union Berlin on loan as he struggled to find a new club to re-establish himself.
The goalkeeper left Liverpool having experienced plenty of “ups and downs” and had stated he was “ready mentally and physically” for his next challenge.
Newcastle hold an option to extend Karius’ deal until the end of the season.
Darlow is set for a spell on the sidelines after sustaining a torn ankle ligament in training on Wednesday and with Martin Dubravka on loan at Man United, it has left Eddie Howe short.
Karius will now act as deputy to Nick Pope, with Mark Gillespie also in the goalkeeping ranks for Howe.
The German’s last competitive appearance was in February 2021, with Union Berlin, and the move to Newcastle represents a valuable opportunity for Karius to get his career back on track.
The 29-year-old made 49 appearances for the Reds during his six years on the club’s books.