The 39-year-old has signed a one year contract at Anfield and will be the third choice keeper for Klopp behind Mignolet and new signing Loris Karius. Manninger was released from FC Augsburg at the end of last season.
Recently, Liverpool loaned out the likes of Bogdan and Ward to free up a space for the Austrian keeper.
Manninger said: “Honestly, it was a surprise. After these years, in Italy and the last four years in Germany, I hadn’t given up, I hadn’t said ‘that’s it’, I still waited for the phone call.