1240 GMT September 18, 2019
Manninger, who won the last of his 33 caps for Austria in 2009, was released by German side FC Augsburg last month, BBC reported.
He has been training with Liverpool and now becomes its third-choice keeper, behind Loris Karius and Simon Mignolet.
"I've still got something left, for sure," said Manninger, who spent five years at Arsenal but has not played in the Premier League since May 2001.
Asked about his return to English football after such a long absence, he told the Liverpool website, "After these years, in Italy and the last four years in Germany, I hadn't given up, I still waited for the phone call.
"Obviously it's probably not the time for me to push for the next three or four years to get a place in the goal.
"Time says it's not possible to play 50 games a year. But if I'm called up, I'm definitely there for games, I will play."
Liverpool has signed six other players this summer so far — Mainz goalkeeper Loris Karius, Schalke center-back Joel Matip, Red Star Belgrade midfielder Marko Grujic, Southampton forward Sadio Mane, FC Augsburg defender Ragnar Klavan and Newcastle midfielder Georginio Wijnaldum.